I have a love-hate relationship with Dungeons & Dragons, 5th Edition. On the one hand, I like the actual the game. The mechanics. The tools. All that crap. Oh sure, there’s some crap too. Every game has crap, but for the most part, I really do love the game. But I hate the f$&%ing books. The books are garbage. Utter garbage. And people don’t see it because most people reading the books are already gamers. They already know how the game works. So the PHB didn’t really have to teach much. It didn’t have to explain much. Instead, it just had to tug at your nostalgia heartstrings. It just had to cram as many references to bygone eras and bygone campaigns as possible. Bam! Instant hit.

The designers of D&D are very good at creating a game. I will give them that. Except for some bizarre-a$% oddities like the fact that magic missile is not intended to be an attack and dispel magic only actually works on spells and not on any magical class ability. But every game has it weirdities. And you can live with those. The problem is they are really bad at explaining the game. Or presenting it. The encounter building math is complicated and confusing, sure. But it doesn’t have to be. The problem is that it is poorly explained. And even worse, customizing monsters is atrociously explained.

Here’s the deal: D&D actually has a really good mathematical system behind its monsters. Or as good as it can be given how to came to pass, but I won’t get into that. End of the day, there is some really solid logic behind monster building. And I love it. Because I’m a gamer and I’m an accountant and I love rules and I love systems. I feel the way about RPG systems that Kate McCartney feels about baking. “It’s just a heap of rules and I’m good with rules. In fact, if this was a different time in history, there’s a very good chance I would have let war crimes happen.”

The problem is, on top of the beautiful mathematical system, there’s a well-documented and exhaustively explained process that is utter crap. Not only are the steps in the wrong order, but the important bits are buried under mounds and mounds of garbage. For example, the DMG advises that the first thing you do when creating a monster is come up with a Name, Size, Type, and Alignment. Then, assign Ability Scores. And then, THEN, think about what you’re going to do with the monster in the game. Like, hey, how powerful do you actually want this thing to be?

Worse still, the system basically says “just make a monster all willy nilly and then, hey, see what Challenge it comes out with. We can always fix it later.” And there’s no f$&%ing reason for that trial and error bulls$&%. Because if you understand how all the pieces fit together, you can figure out how to do some pretty cool stuff. Like, amazingly cool stuff. But that would require having the system explained. And delineating a process is NOT the same as explaining a system. Delineating a process does not empower people. Explaining a system does. Because then they can deviate from the process and still understand what the hell they are doing!

So, I’m going to teach you a better way to build a D&D 5E monster. Which shouldn’t surprise you, because I promised you that last week as part of my unofficial “Monster Month.” We’re going to be spending the whole month on building D&D monsters. In fact, we are going to spend more than a month because, surprise surprise, I’m going to give you a bonus article.

In THIS article, we’re going to analyze the rules that make 5E monsters work. I’m going to try to explain how it all works and how things all fit together better than the DMG did. And I’m doing that so that, NEXT article, we can actually find a process for building awesome D&D monsters. And then we’re going to do the same damned thing in Pathfinder.

Let’s Get Dissecting!

First and foremost, I’m here to teach you how to build complete and legal monsters. I explained why already, so I’m not going to do it again. If you want to fudge s$&% or reskin s$&%, go do that and get the hell off my site. The monsters I am going to teach you to build are mechanically rigorous. They could sit in a published product and be as accurate as any other goddamned thing in the D&D rules.

That means, you’ve got to understand the system. And I mean you have to really understand it. You’ve got to have a grasp on how all the little pieces affect each other. Because, next week, I’m going to show you how powerful the whole damned system is if you ignore the way WotC tells you to use it. Some of this information might be remedial if you’ve made a good study of the process already, but that’s just tough s$&%. You have to learn to walk before you can flee from a rampaging gorilla-demon while lemur people are flinging rocks at you.

What follows is a whirlwind tour of the bits of pieces that make the monsters work mathematically.

Proficiency Bonus

Every monster has a Proficiency Bonus based on it’s challenge, as you can see in the first column of the table on DMG 274. This bonus is added to attacks with which the monster is proficient, Saving Throws with which the monster is proficient, and skills with which the monster is proficient. That isn’t optional. That’s a rule.

Well, sort of. You can deviate from that. For example, some monsters actually have a sort of expertise, where they add double their Proficiency Bonus to a skill. This isn’t explicitly called out in the stat block, but it does happen. For example, check out the Grell on MM172. It is proficient in two skills, Perception and Stealth. It has a Stealth modifier of +6. It’s Dexterity modifier is +2. It’s Proficiency Bonus should be +2 based on it’s Challenge. So where is the extra +2 coming from? Well, some PC class traits, racial traits, and feat traits allow them to add double their proficiency bonus to specific checks. Clearly, that’s where it’s coming from. Grells are Stealth and Perception experts. They use double their Proficiency Bonus for those skills.

On the other hand, in monsters seem to ALWAYS be proficient with their attacks. I haven’t spotted any exceptions to this rule. It’s a strong enough trend that, recently, when I needed to create a creature with an attack roll lower than what the math allowed, I wrote a special trait to explain it.

Lummox. Erug is a clumsy brute who swings wildly with his weapons. He does not add his proficiency bonus to weapon attacks.

I don’t know if I had to that for my clumsy ogre brute, but I like to be thorough. When I deviate from the system, I like to explain it. That’s my design philosophy.

So Proficiency Bonus is always added to attacks and always added to Saving Throws with which the monster is proficient. A monster also adds its Proficiency Bonus to skills with which it is proficient. It can also add twice the proficiency bonus to give it expertise in certain skills.

What that means is that the Proficiency Bonus is pretty central to the monster’s attacks. And because it based on Challenge, it’s pretty fixed. You’re always going to have to account for Proficiency Bonus. But, as you’re going to see, that can make things very complicated.

Attack Bonus

When a monster can make an attack, there’s two important statistics: attack bonus and damage. A monster’s melee attack bonus is equal to its Proficiency Bonus plus its Strength modifier. A monster’s ranged attack bonus is equal to its Proficiency Bonus plus its Dexterity modifier. IN GENERAL. See, this is where things get a little weird. If a monster is using a weapon, it follows the rules for those weapons. A monster using a scimitar can substitute its Dexterity modifier for Strength because a scimitar is a Finesse weapon. A monster throwing a spear uses its Strength modifier, not its Dexterity, because it is a Thrown weapon. So, monsters using manufactured weapons follow the rules for manufactured weapons.

Now, when a monster uses a natural weapon, like claws or teeth or tentacles or tail spikes, things can get a little complicated. Or they might seem to. Except they don’t really. What you’re really doing when you give a monster a natural weapon is you’re inventing a weapon. Take, for example, the bite of a flying snake (MM 322). That’s a melee attack, and if it used the creature’s Strength modifier (-3), the attack bonus would be -1. But it’s +6. Where is that +6 coming from? Well, the creature’s Dexterity modifier is +4 and it has a proficiency bonus of +2. That’s the +6. Which tells us the creature’s bite is a Finesse weapon. See? Simple.

Manufactured weapon attacks follow the weapon rules for attack bonus, including the rules for Finesse melee weapons and Thrown ranged weapons. Natural weapon attacks follow the same rules, but you can decide that a natural weapon is Finesse or Thrown pretty freely. So, if I have a high-Strength, low-Dexterity quillbeast that fires quills, I can designate those as the equivalent of a Thrown attack and calculate based on the Strength score.

But, obviously, all of this should make some sort of logical sense. It’s one thing to designate claws and bites as Finesse because precision can be more important than power there, but when it comes to a slam attack, that’s a little harder to justify. You have to decide where your lines are. Just remember that players encountering your creations and other GMs using your creations will expect them to be logically consistent and behave accordingly.

Damage

Let’s talk a little bit about damage. For manufactured weapons, damage is pretty easy. It’s listed in the weapon table. A kobold wielding a shortsword, a goblin wielding a shortsword, and an orc wielding a shortsword all deal 1d6 piercing damage. As for the bonus on the end. The damage bonus is always equal to the SAME ABILITY SCORE MODIFIER as the attack bonus is based on. If it’s a Finesse weapon, and you use Dexterity for the attack bonus, you have to use Dexterity for the damage bonus. A Thrown weapon uses Strength for the damage bonus.

Now, larger creatures using manufactured weapons deal more damage. A large creature using a manufactured weapon deals double the damage dice. But the bonus damage from the ability score remains the same. So, an orc wielding a short sword might deal 1d6+2 damage. If the orc suddenly grew to large size (all else being equal), it would deal 2d6+2 damage. Not 2d6+4. Huge creatures deal three times the damage dice. So the orc that kept growing would deal 3d6+2 damage and then 4d6+2. That assumes the weapon grows with the monster.

Manufactured weapons are easy to calculate. But natural weapons are more open-ended. You can – in theory – assign any damage die you want to a natural attack. But, in general, it’s a good idea to follow the convention of number of dice based on size unless you have a very good reason. A small creature’s bite should probably not deal 4d6 damage. Players expect the size of a creature to say something about its damage and plan accordingly.

Just like with manufactured weapons, natural weapons always add a bonus based on the same Ability Score modifier as the attack roll. But here’s where things also get a little different. Attacks that deal damage types other than bludgeoning, piercing, and slashing, such as the lightning breath of a behir (MM 25) or the fiery touch of a magmin (MM 212) don’t generally add a damage bonus at all. They are based purely on a dice code. This is a good rule to follow. When an attack is based on some sort of odd damage or energy type, don’t add an ability modifier.

Saving Throw DC

Some attacks and abilities don’t involve attack rolls at all. Instead, they allow the target a Saving Throw to avoid or mitigate the effect. In those cases, it’s up to you, the designer, to decide which Ability Score determines the Saving Throw DC and which Saving Throw the target rolls. Generally, though, there seem to be some rules of thumb. If a creature is a spellcaster, it has a spellcasting ability based on the rules it uses for it’s spells. For example, a creature that casts spells as a cleric uses Wisdom as it’s spellcasting ability. But some creatures aren’t spellcasters. They just have some sort of innate ability that requires a Saving Throw. Poison and disease are the most common mundane abilities. Breath weapons and fear auras are examples of magical effects.

All Saving Throw DCs are calculated the same way. They are 8 plus the Proficiency Bonus of the creature plus the relevant Ability Modifier. Like Attack Bonus, there’s no deviating from this rule (unless you build a special trait to explain the exception). As for what Ability Score to use? It’s up to you, but there do seem to be some patterns. Physical effects are based on Constitution. That includes poison, disease, and breath weapons. You can reverse engineer a lot of the examples from the Monster Manual and discover that they seem to all be based on Constitution. Though, that is an educated guess in some cases because a creature has the same Constitution and Strength modifier, for example. And while the pattern doesn’t seem universal, it does seem pretty strong. So I prefer to follow it unless I have a really good reason not too. For example, a Knockdown ability might use a Strength-based DC.

Magical effects seem to be based on Charisma. But there’s something weird going on in a few creatures and the pattern may not be as strong here. The Intellect Devourer is absolutely the strangest case (MM 191). It’s Devour Intellect ability has a Saving Throw DC of 12, but the only way that could possibly work is if it is actually based on Dexterity. That might make sense if the the thing were a brain-eating face hugger, but Devour Intellect is a psychic ability. It just goes to show that even the designers sometimes f$&% up. Because I can’t believe that was done intentionally.

In general, my rule of thumb therefore is to base physical effects on Constitution and mental or magical effects on Charisma unless I have a really, REALLY strong reason to do otherwise.

Hit Points

A creature’s hit points are determined by a dice code. And that dice code is very simple. It is always some multiple of one die plus the Constitution modifier. You can sort of think of a monster as having a “level,” but only for hit point purposes. For each “level,” it rolls one hit die for its hit points and adds its Constitution modifier. This is a solid rule.

If you have a monster with a Constitution modifier of +3, it can have 1d6+3 hit points or 2d6+6 or 3d6+9 or 4d6+12. But you can’t have 2d6+3 or 1d6+6. The number of dice rolled is always also the number of times you add the Constitution modifier. If you roll 6 dice, you have to add six times the Constitution modifier. No exceptions. Check your Monster Manual and you’ll see 99% of the creatures follow this rule. I want to say it’s 100%, but I might have missed one or two that the designers screwed up. So I can’t swear to it.

Note, however, that the number of times creatures roll for hit points has NOTHING to do with its challenge. If you look back at your trusty table on DMG 274, you’ll notice that the total hit points is based on the Challenge. Or the Challenge is based on the hit points. It goes both ways.

The die you roll is determined entirely by the creature’s size. Small creatures ALWAYS use a d6. Large creatures ALWAYS use a d10. Notice this is true even of humans with apparent class features (MM 342-350) and when adding class levels to a creature (DMG 283). It’s a firm f$&%ing rule.

Extra Credit: Average Dice Codes and Hit Points

One of the most useful things you NEED TO KNOW when creating monsters is how to compute an average dice roll. Because the HP in a stat block (and the damage) are given as average numbers with dice codes in parenthesis. E.g.: Hit Points 90 (12d8 + 36). It’s important to know how to convert from one to the other.

The average roll on any die – for reasons of probability I’m not going to try to explain right now – is half the number of faces on the die plus one half. A d4 yields 2.5. A d8 yields 4.5. And so forth. So, how do you get to 90 from 12d8 + 36? Well, the average of a d8 is 4.5. Multiply 4.5 by 12 and you get 54. Then add 36 on top of it.

Note, also, that you know what that creature’s Constitution modifier is, right? It’s +3. Why? Because we add the Constitution modifier once for each die we roll. We also know the creature is medium size because medium size creatures always roll a d8 for hit points. Now, I’m going to a bit of mathematical gymnastics that will be VERY important. Watch carefully.

If a medium size creature has a Constitution modifier of +3 that means that it’s HP are going to be some multiple of 1d 8+3. Right? It’s going to roll 1d8+3 some number of times to determine its total hit points. In the case of the creature above, it rolled that 12 times. Got it? 12d8+36 is 1d8+3 12 times. Why do I bring this up? Well, because you’re going to need to figure out how to roll a specific number.

For example, I might be designing a creature and need to roll between 80 and 100 hit points. If the creature is small and it has a Constitution modifier of +1, I can actually figure out exactly the dice code I need to get in that range. Follow the logic. Small creatures roll a d6 for hit points. So this creature is going to roll 1d6+1 some number of times to determine its hit points. What’s the average roll? Well, the average on 1d6 is 3.5 (half of 6 plus a half), so the average of 1d6+1 is 3.5+1 or 4.5. So, if I take, say, 85 and divide it by 4.5, I get 18.8. That means I need to roll 1d8+1 about 18 times to get in the ballpark of 85. In this case, if I multiply 4.5 times 18, I get 81. Perfect. So, this creature has 81 (18d6+18) hit points.

Let’s try another one, because this is SUPER IMPORTANT. It’s the most mathy thing you have to do when making creatures. A large creature with +3 Constitution modifier and you want 160 – 175 hit points. Try it yourself.

Did you try it?

Okay, here’s my answer. Large creatures roll d10s, so the creature is going to roll d10+3 some number of times. The average on the die is 5.5. Add 3 and you get 8.5. 160 divided by 8.5 is 18.8. Well, multiply 18 times 8.5 and you get 153. A little too low. So the smallest would be 161 (19d10+57). The largest would be 170 (20d10+60).

It’s really important to be able to work back into a dice code from a hit point target.

Armor Class

If a creature wears manufactured armor, they gain the Armor Class formula described for that armor. Put a beast in Studded Leather armor, their AC is 12 + Dexterity modifier. Put them in Breastplate, its 14 + Dexterity modifier up to 2. And if they wear Plate, their AC is 18. Give them a shield and they increase their AC by +2 when using the shield. Simple.

But when it comes to natural armor, you have to understand the philosophy behind D&D AC. There are two TYPES of Armor Class: formulas and bonuses. Shields and shield spells are good examples of AC bonuses. Whatever your armor, you get a +2 to your Armor Class. Or whatever. Same with cover. Cover grants you a bonus to AC. But actual ARMOR grants you an AC formula. It tells you how to calculate your AC. Leather armor grants you the formula AC = 11 + Dexterity modifier. Plate grants you the formula AC = 18. End of story. You can add bonuses over the top of it, but Armor Class formulas don’t stack.

Why do I bring this up? Because if you very thoroughly examine the Monster Manual, you will discover that creatures have EITHER natural armor OR manufactured armor. They never stack one on the other. For example, look at the giants (MM 154-155). Some of them use natural armor, some of them wear manufactured armor. But there’s really nothing differentiating their hides. Thus, we can conclude that they all actually have natural armor. Some of them just wear manufactured armor over it. And since the manufactured armor grants an Armor Class formula and the natural armor doesn’t seem to be stacked with the natural armor, we can conclude natural armor grants an AC formula and doesn’t stack with worn armor.

That means, when you are giving a creature natural armor, you’re free to invent the armor any way you want, just like when you give a creature a natural weapon. Technically, you’re creating a new piece of equipment. How does this work? I have no f$&%ing clue. Because once it’s an AC formula, it could be anything. For example, check out the marilith (MM 61). She is clearly wearing a breastplate in the picture, which should make her AC 14 + Dexterity modifier (max 2) or 16. But her AC is 18 with natural armor. What are we to conclude from that? Is her natural armor like light armor (AC = 13 + Dexterity modifier), like medium armor (AC = 16 + Dexterity modifier (max 2)), or like heavy armor (AC = 18). Any one of those three solutions are valid. If I wanted to be really nitpicky, I’d point out that her natural armor couldn’t be like light armor because the breastplate she is wearing in the picture would then limit her Dexterity modifier (logically) and she wouldn’t lose that restriction just because she can’t benefit from the breastplate. Thus, I could use that to say that natural armor follows the same rules for manufactured armor right down to being light, medium, or heavy. But anyone could also argue the picture is just a picture.

In the end, though, it doesn’t matter. Just know this: either a creature has natural armor and you can set the AC to be anything OR the creature wears manufactured armor and you use the AC formula to calculate it. And you can always add a shield.

Also note that you can invent your own manufactured armor types. For example, Armor Scraps is a type of light armor that grants an AC of 11 + Dexterity modifier. How do I know? Because I looked at the skeleton (MM 272). How do I know it’s not medium armor that grants AC 11 + Dexterity modifier (max 2) or heavy armor that grants (AC 13)? Well, to be fair, I technically don’t. But there IS a pretty solid pattern for AC vs. weight of the armor. For something to grant only AC 11-13, it’s probably light. I’m guessing at this, but the pattern seems pretty clear.

Saving Throws

Most monsters are not proficient with any Saving Throws, which is why most monsters don’t have a Saving Throws line in the stat block. The only time a monster gets an entry for Saving Throws is if they have one or more Saving Throws that add something OTHER than the ability bonus. In general, though, you can grant a monster proficiency with up to two Saving Throws without really having a drastic impact on the monster. When you do so, the monster’s Saving Throw bonus with that Saving Throw is equal to the Ability Modifier plus the monster’s Proficiency Bonus. Should you do this? The DMG suggests you do it to bolster a low Ability Score, but frankly, I think that’s actually a bad suggestion. Remember that players are attentive. If they notice a creature is not very nimble, they are going to use spells with Dexterity saves over those with Constitution or Strength or Wisdom saves. At least, some will. And they deserve that opportunity. And a creature’s poor ability score SHOULD be kryptonite. It gives the creature variety and rewards the players for attentiveness and tactical thought.

In truth, I rarely make creatures proficient with Saving Throws except to emphasize something special about the creature or to communicate something about its nature. Often, that means I use Saving Throw proficiencies on top of HIGH ability scores more often than on low ability scores. Or on mediocre ability scores. For example, I recently made a soldier devil. It’s basically got infernal levels of dedication. So I gave it a proficiency with Wisdom saving throws to emphasize the fact that this thing doesn’t waiver from its duty or nature.

Actions, Reactions, and Traits

This is a big, complicated thing. Obviously, we’ve already attacks and damage dealing. But what about all those other neat things? Like a monster that can constrict with its tentacles or gain advantage on an attack when it has adjacent allies or a monster that can breathe a cone of burning acid-fire? I mean, surely, that’s where the art of monster building comes into its own, right?

Well, yes. Yes it does. And it is both an art and a skill. It requires a lot of finesse. And I can’t tell you how to invent new traits. You’re going to have to bring your own creative spark to the forge. But what I can tell you is that there is actually a logic to how those traits affect the monster. That is to say, there is a logic to balancing those traits, to deciding how powerful they are. But in order to talk about that, we need to talk about challenge and how challenge is actually calculated.

Challenge Rating

Challenge Rating determines how many XP a creature is worth, right? Which means it also determines how dangerous it is. A Challenge 5 creature is a medium difficulty encounter for four 5th-level PCs. Remember that, by the way. That’s the baseline. A Challenge X creature is a medium difficulty encounter for four X-level PCs.

But Challenge is actually a bit of a paradox and WotC isn’t going to help you. THEIR advice is to build a f$&%ing creature and then use their system to figure out the Challenge. Which is backwards. As I said, you build the creature to fill a role in your game. And part of that role is “what level are the stupid PCs that are going to fight this thing?” And that means, you can’t build a creature and then get surprised by the Challenge.

But where things get complicated is that Challenge is determined by five basic factors: AC, HP, Attack, Save DC, and Damage. And Attack and Save DC include a Proficiency Bonus. And Proficiency Bonus is based on Challenge. So, Challenge determines Proficiency Bonus which determines Attacks and Save DCs which determines Challenge. Isn’t that a delightful f$&%ing circle?!

Except it isn’t. Not really. Because the other thing that determines Attacks and Save DCs – along with every other f$%&ing thing that you can’t freely invent – are Ability Scores. Right? Attack is a combination of Ability Score Modifier plus Proficiency Bonus. And THAT is why I say you do the Ability Scores last. Because that is where you have final control over the numbers. See? I’m not crazy!

But let’s talk about how Challenge is determined.

A monster’s Challenge is the average of its Defensive Challenge and its Offensive Challenge. Take the two numbers, add them together, and divide by 2. If the creature has a Defensive Challenge of 4 and an Offensive Challenge of 1, its Challenge is 3 ((4 + 1) / 2 = 2.5, round up to 3).

So, how do you determine Offensive and Defensive Challenge?

First, you open your DMG to page 274. You start by finding the creature’s hit points on the table. And whatever row that is, that’s the starting Defensive Challenge. Now, look at the Armor Class on that table. Is your monster’s AC above or below that? For every two points above the AC on the table, increase the Defensive Challenge to the next highest one. For every two points below the AC on the table, decrease the Defensive Challenge to next lowest one.

For example, you have a creature with an AC 17 and HP 66 (6d6+12). 66 HP puts it in the Challenge 1/2 row, right? But a Challenge 1/2 creature is expected to have an AC 13. My AC is 17, four points higher. So I have to bump up the Defensive Challenge by moving down two rows. I go from 1/2 to 1 to 2. My Defensive Challenge is 2.

What about Offensive Challenge? Well, that starts with the average damage per round. For most creatures, you can simply look at their most damaging attack. Whatever the fixed damage is, that’s the damage output. Find that on the table. Then, look at the Attack Bonus or the Save DC (depending on whether it uses an attack roll or allows a save) on the table. And just like with AC, you adjust the Offensive Challenge up or down for every two points by which the monster’s Attack Bonus or Save DC differs from the one given on the table.

For example, you have a creature that attacks with a shortbow. It has +5 Attack and does 6 (1d6+3) piercing damage. The damage makes it a Challenge 1/2 . But its Attack Bonus is two points higher than the expected +3 for a Challenge 1/2 creature. So we have to increase the Challenge once. The Offensive Challenge is 1.

What’s the final Challenge for that creature? It’s 2. We add the Offensive and Defensive Challenge, divide by two, and then round up. 1 + 2 is 3, divided by 2 is 1.5, rounded up is 2. Got it?

Now, just to prove you’re really understanding the system, what can you tell me about this creature from the stats I gave you? AC 17, HP 66 (6d6+12), Shortbow Attack +5, 6 (1d6+3) piercing damage. Take a moment and figure out as much as you can.

And here’s a bonus question. Assume the creature is not proficient with Stealth and it tries to hide. Can you tell me what it’s going to roll?

Okay, first, we know the creature is small, right? It’s rolling a d6 for hit points. And it has a Constitution modifier of +2, right? Because it’s rolling 1d6+2 six times to determine its Hit Points. Now, because we know its Challenge is 2, we know its proficiency bonus is +2. It’s using a short bow, a ranged weapon, so it’s using Dexterity. Its attack is +5. If we take off the proficiency bonus, we’re left with +3 that is coming from its Dexterity, right? Which makes sense because its damage bonus is +3 and is always based on the same stat as the attack. What about its armor? If it is small and using a bow, it might have natural armor, but it’s more likely it’s wearing manufactured armor. What might it be wearing? Well, it has a Dexterity of +3 and we know it’s not using a shield because it needs two hands for that short bow. So, what are its armor options? Well, if it’s wearing heavy armor, it’s wearing splint mail. That’s easy enough. AC 17. If it’s wearing medium armor, it’s got to be half plate. Why? Medium armor limits the Dexterity modifier to +2. Thus, it’s got an AC of 15 + Dexterity modifier (max 2). And that’s half plate. Light armor means… well, that means it’s not wearing light armor. It would need an AC of 14 + Dexterity modifier to get up to light armor.

What about the Stealth check? Well, if it has natural armor, it has Stealth +3, right? Just the Dexterity. But if it has any sort of manufactured armor at all, and it probably does, that means it’s rolling with disadvantage.

How’d you do?

Try another one, just to check that you have it right. Hit Points 120 (16d10+32), Armor Class 11, Slam +5 attack, 22 (4d8+4) bludgeoning damage. Figure out the final Challenge. And then, as a bonus question, tell my why that monster isn’t legal unless it has some weird trait affecting its stats.

Got it?

HP 120 means a Defensive Challenge of 4. But the AC for a Challenge 4 is 14. This creature is 3 points too low on the AC. So we have to bump down the Defensive Challenge from 4 to 3.

Damage 22 means the Offensive Challenge is 3. And the attack should be +4. It’s +5. That doesn’t deviate by 2 points either way, so its Offensive Challenge is 3.

That means the creature’s final Challenge is 4.

Why isn’t the creature legal? Well, at Challenge 4, it’s got a proficiency bonus of +2. So, its attack has an ability modifier of +3. But the damage has a bonus of +4. Since those should be based on the same ability modifier, something funky is going on. In this case, it might have a trait that gives it a +1 to damage. That’s a weird trait, but whatever.

Where Challenge Gets Crazy

Now, you might notice that this whole Challenge thing is really simple, right? Hit Points, Armor Class, Damage, and Attack or Save DC. Done and done. But that’s just not possible, is it? It can’t be that simple, can it?

Well, no, it can’t.

For example, kobolds have a trait called Pack Tactics. They gain advantage when they have allies adjacent to their target. And advantage equates to a +4 to attack, in general. Surely that’s got to be factored in right? And what about a creature that has a lot of damage resistances, like undead. Hell, even a creature that can fly is more challenging to deal with than a creature that can’t. How do you handle all of this stuff?

Well, the DMG uses a concept called “Effective” stats. For example, if you have a flying monster whose Challenge is 10 or below, most PCs are going to have a difficult time with that creature since PCs below level 10 usually can’t fly. The creature can stay out of reach and limit their melee capabilities. So, DMG 279 explains that we increase the Effective AC by 2. Now, that’s not an AC bonus. Its AC doesn’t change. But when we determine the Defensive Challenge of the creature, we imagine the AC is two points higher to account for the flying.

The DMG lays out a lot of rules of thumb from DMG 275-279. If the creature is resistant to many damage types, increase the Effective HP by a factor based on its Challenge (x2, x1.5, or x1.25). If it has a lot of vulnerabilities, decrease the Effective HP by a factor. If the creature can fly, increase the AC by 2. If the creature is proficient with three Saving Throws, increase the Effective AC by 2. If it’s proficient with five Saving Throws, increase the Effective AC by 4.

As you build more monsters, you get used to these things. In addition, different Traits also change different Effective stats. And there’s a big list of Traits from DMG 280 to DMG 281. Each Trait tells you what monster it comes from and what the overall effect on the creature’s Effective stats are. So, if I give a tentacle monster the Constrict trait that Constrictor Snakes have (MM 320), I increase the snake’s Effective AC when I determine the Challenge. Again, I don’t change the ACTUAL AC. I just figure the Challenge as if the AC was higher.

Let’s create a monster. I’m going to create a thing. I’m going to create enchanted ceramic wolf monsters. Because they are ceramic, they are resistant to slashing and piercing weapons. Bludgeoning weapons deal normal damage, though. Because they are wolves, they gain advantage on attacks when they have adjacent allies, so they have Pack Tactics. In addition, they are very fast. As a bonus action, they can move toward a hostile creature. I’m stealing that from orcs (MM 246). It’s called Aggressive.

Ceramic Guardian Wolf, AC 14, Hit Points 22 (4d8+4), Bite attack +5, 6 (1d6+3) piercing damage. Resistant to piercing and slashing. Pack Tactics. Aggressive.

How the hell do we figure the Challenge on that? Well, we do it one step at a time. First, what are the effects of all of our traits? Multiple resistances mean we have to double the Effective HP. Even though the creature has 22 HP, we’ve got to figure the Challenge as if it had 44. Next, Pack Tactics effectively increases the attack bonus by 1, so we figure the Challenge as if the Creature has an attack of +6. And Aggressive increases the effective damage by 2. That means, even though we aren’t changing any of the stats, we’re figuring the Challenge as if the creature had AC 14, Hit Points 44, Attack +6, Damage 8.

Open up to the table and figure it out. Did you get Defensive 1/4, Offensive 1? Great! And what did you decide is the average? Did you decide the final Challenge is 1/2 or did you call it 1? Tough call, right? The average of 1/4 and 1 is 0.625 which is closer to 1/2 than 1. Do we always round up? Or do we round off? Honestly? That’s your call. The books aren’t totally clear on the better approach. I generally follow normal rounding rules, rounding down or up depending on the result. But if you always round up, you’ll probably be fine. It’s your call.

Okay? So that’s Traits and special abilities. But that isn’t all. Because there’s another monkey wrench here.

Damage Output per Round

Some creatures have multiple attacks. Some creatures can attack several times in a round. Some creatures can cast spells. Some creatures use area attacks like breath weapons. What do all of these things have in common? They all mean the creature’s damage output varies from round to round.

First, let’s talk about figuring a creature’s TOTAL damage output. Let’s say I have a flaming lava statue that swings a fiery lava axe. The thing (the statue, not the axe) is so hot that it has a damaging aura. At the start of each turn, the statue deals 3 (1d6) fire damage to everyone within 5 feet of it and anyone who hits with a melee weapon suffers 3 (1d6) fire damage. Second, let’s say its lava axe does 7 (1d8+3) slashing damage plus 3 (1d6) fire damage. Third, let’s say I’m a real bastard and I give the thing multiattack. It can swing its axe twice in a round.

How much damage does this thing actually do in a round (assuming it hits, because we always assume it hits because the Offensive Challenge is adjusted by Attack Bonus or Save DC later). First of all, that axe does 10 damage per swing and it’s going to get swung twice in a round, every round. So, that’s 20 damage right off the bat. Now, in addition, we can assume that the statue is going to keep at one PC adjacent to at all times, so it’s always going to be doing 3 fire damage. And someone is probably going to hit with a melee weapon every round and soak up the damage, so they will also take 3 damage. So, the statue’s effective damage output per round is 26. Yowza!

It’s important to include ALL sources of damage when you figure the damage output of a creature.

At the same time, we also always assume the creature will be using its best option. So, if I have a little goblin wizard with a ray of frost cantrip (3 (1d6) cold damage) and a dagger (1 (1d4-1) piercing damage), we assume it will use its ray of frost every round. The damage output is 3. It will only use that dagger in an emergency. Or not at all.

What about area attacks? What about a creature that can breathe fire? In general, we assume that an area attack will hit two creatures at a pop. The monster generally won’t waste the area attack unless it can hit more than one target. So if my statue can spew a cone of lava that does 10 (3d6) fire damage, we assume it’s going to hit two people and deal 20 damage per round with that.

And that brings us around to recharging abilities and different attack modes. Imagine a tentacled strangler that has a tentacle attack that does modest damage when it hits, like 4 (1d6+1). But it can grapple a target with that attack. If it starts a round with a creature grappled, though, it can deal 7 (2d6) damage by strangling. How do you figure the damage output on that?

Or, what about a creature that has a limited ability. Like, what if a creature can shoot eye lasers once per day that do 14 (4d6) fire damage. What about a creature with a breath weapon that does 10 (3d6) fire damage to an area, but it has Recharge 5-6? How do we do those damage outputs?

Well, we have to make some assumptions. Generally, we try to figure out how much damage the creature will do for each of the first three rounds and then take the average. And we usually have to make some assumptions to do this.

For example, take the fire breath. Area attacks hit twice, so it deals 20 damage in the first round. But that thing has a recharge. It probably won’t be available again for a while. Basically, with a recharge 5-6, it will be available one out of every three rounds. The other two rounds, the creature will have to use its bite attack 5 (1d6+2). So its damage output over three rounds is 30, which means its average damage output per round is 10. And THAT is what we use to figure the effective challenge.

What about the eye laser once a day? Same story. It opens with 14 damage. And then it uses its longsword for 7 (1d8+3) damage for the next two rounds. That’s 28 total damage output for three rounds, or 9 damage per round.

And the strangler? Well, assume it attacks on the first round and hits. It does 4 (1d6+1) damage. But on the second round, it probably has a creature grappled so it will deal 7 damage (2d6) damage. But then, the creature will probably have escaped (it’s got two chances by this point). So the third round, it will deal 4 damage again. That’s 15 damage over three rounds or 5 damage per round.

As for what Attack or Save DC to use when you have to average damage over several rounds? You use the one that it will use the most. The fire breather is using its longsword twice and its breath once, so use the longsword Attack Bonus not the fire breath Save DC.

Now, total Damage Output per Round is an extremely important concept. Why? Because, more than any other factor, damage (and hit points, which you can think of as the inverse of damage) determines the actual level of a thing. See, D&D is built on this concept of Bounded Accuracy. What that means is that the designers wanted to keep things like attacks, saves, and ability checks from ballooning to ridiculous numbers. You’ll never roll 1d20+25 for anything in D&D. At least, that’s the idea. In reality, at very high levels, you could conceivably roll an attack roll as high as 1d20+15 (+5 ability score, +5 proficiency, +3 magic item, +2 from some random bonus or circumstance). But for practical purposes, most games don’t get that high.

What that means is that damage and HP are where most of the leveling happens. And you can see this. Notice what happens at Levels 5, 11, and 17. Go ahead and check your cantrips. Check Ray of Frost. Check Fire Bolt. Check Magic Missile. Notice anything?

Yeah, the damage output doubles. And if you look carefully at all the various classes, you’ll notice most of the classes get double damage at that point. Damage and HP are the primary measurements of level is what I’m saying.

But the thing is, as you build monsters, you’re going to find it’s harder and harder to pump the damage to the expected levels. And you have to start pulling more and more tricks to do it. The most common trick – which we will talk a lot about next time – is a little friend I like to call Multiattack. Notice how many monsters in the DMG have Multiattack. Especially monsters with high Challenge who use weapons. Other tricks include bonus actions, adding extra damage types onto attacks, area attacks, auras, and on and on and on.

Levers and Knob

Okay, so why this deep analysis? Let’s end with that question so we don’t have to start with it next time. Because I’m going to start with getting nostalgic for the good old days of 4E. I’m not s$&%ing you. What’s the point of knowing all of this?

Well, it tells you what you can and can’t control in monster creation. For example, you have NO CONTROL over proficiency bonus. It is determined entirely by the Challenge. And it is always added to certain things. Which is why, again, I say its bats$&% insane for the DMG to leave Challenge for the end. Challenge should be the first thing you pick. Because Challenge also tells you where the HP, AC, Damage, and Attack Bonus have to fall. So you pick your Challenge with care.

BUT, now you also understand that Challenge has two components. Offensive and Defensive. And Offensive Challenge starts with Damage and then is modified by Attack and Save DC. And Defensive Challenge starts with Hit Points and then is modified by Armor Class. That understanding is central to what we’re going to do next time. So make sure you understand it.

MOREOVER, you also understand what you CAN Tweak. You have the LEAST control over Attack Bonus. Why? Because it’s mostly Proficiency Modifier and if you tweak the Ability Modifier, you’re also f$&%ing with the Damage. And maybe the Armor Class if the creature is using a Finesse attack and the equivalent of light armor (natural or artificial). With manufactured weapons, you trade damage for potential Armor Class as well. Two-handed weapons do the most damage but they cost you a shield.

MEANWHILE, traits, special abilities, even movement modes affect the Challenge of the creature. Which means you’ve got to take them into account. Giving a monster Nimble Escape seems like a neat way to make an assassin, but remember that creature is boosting its Armor Class and Attack Bonus by hiding every round. And that means, overall, the Challenge of the creature is two levels higher than you think it is. And now you should be able to figure out why I can say that so absolutely. Nimble Attack outright increases the Challenge of any creature by two levels. Every time. Work it out. Check DMG 281 and think through the numbers.

With a keen understanding of all of the levers and knobs you can turn on a monster, you can build any monster you want at any Challenge you want and be confident the numbers are going to work. And even more importantly, you can perform some impressive mechanical sleight of hand to really make your monsters’ mechanics match your flavor every f$&%ing time in an inherently consistent way.

But that’s a discussion for next week.