Screams. The zombie mob forces a creature it is currently grappling to make DC 10 Wisdom saving throw. On a failed save, the creature becomes frightened until the end of its next turn.

The Spreading Dead. Whenever the zombie mob kills a humanoid creature, the slain creature rises again as a part of the mob. Whenever a humanoid dies while sharing a space with the zombie mob, the zombie mob gains an additional d10 hit die and gains 6 hit points.

Zombie Fortitude. Any time the zombie mob would lose one more more hit dice, roll a d4. On the result of a 4, the zombie loses that many hit dice minus 1.

He chooses to use a divine smite on both attacks, dealing 10 slashing 10 radiant damage with each attack. The swarm combines the damage into 40. It loses 4 9 hitpoint HD (36 damage) and ignores the remaining 4 damage.

On the paladins turn, he makes two attack rolls against the swarm. He rolls a natural 5 and 11 which both hit.

18 x 0.25 after rounding down is 4 points of bludgeoning damage before resistances and other damage reducing effects.

Finally, we determine how much of the rolled damage is dealt. The paladin has an AC of 20. The attack roll is 5 lower than the targets AC, putting the damage multiplier in the -1 to -9 range. This means the paladin will take 0.25x the total damage rolled.

Next, it determines the swarm attacks total damage. Batter deals 1d6+1 bludgeoning damage per action expended. The swarm expended 4 of its actions. It rolls 4d6+4 for a total of 18 points of bludgeoning damage.

First, it rolls its swarm attack roll. The swarm rolls an 11 on the d20. Batter has a +1 to hit modifier for each action used when making the swarm attack. It adds the modifier to the rolled result for a total of 15.

It spends one of its actions by using its regular Slam action. It replaces the attack roll with a contested grapple check, and succeeds. It expends its four remaining actions to take the Batter action against the paladin.

On initiative count 20, a Zombie mob at full hit dice attacks a paladin that it shares a space with. It has 5 actions to use on this turn.

When a swarm makes a swarm attack in this fashion, it expends up to one fourth of its avalible reactions. Reactions expended in this way are considered expended actions when determining attack modifiers and damage modifiers to this swarm attack.

A swarm can expend up to one fourth its total reactions to make a swarm attack against a creature when it exits a space both creatures occupy and enters a space the swarm does not occupy.

The following traits apply to all creatures with the swarm creature type:

Managing Swarms

A swarm's hit dice represent individual entities within a swarm. This is great to keep in mind when visualizing and narrating how a party attacks and damages a swarm. Below are some additional tips for managing swarms easier:

Managing Damage Taken. Write down multiples of the creature's hit dice for easy reference. For example, when running a zombie mob write 1/9 2/18 3/27 4/36. These offer a quick reference for number of hit dice lost when the swarm takes damage.

Write down multiples of the creature's hit dice for easy reference. For example, when running a zombie mob write 1/9 2/18 3/27 4/36. These offer a quick reference for number of hit dice lost when the swarm takes damage. Managing Actions. Write down the number of actions the swarm plans on using on each turn next to the turn on the initiative order to help remember how many actions a swarm can take on that turn.

Write down the number of actions the swarm plans on using on each turn next to the turn on the initiative order to help remember how many actions a swarm can take on that turn. Average Damage. Instead of rolling for damage, you can take the average damage of the total dice rolled plus total modifiers. For example, when a Zombie mob expends 8 actions to attack a creature, you can just multiply the average damage (5) times 8 for a total of 40 damage.

Instead of rolling for damage, you can take the average damage of the total dice rolled plus total modifiers. For example, when a Zombie mob expends 8 actions to attack a creature, you can just multiply the average damage (5) times 8 for a total of 40 damage. General Sizes. A swarm of 3-8 medium creatures is typically large, a swarm between 10 and 18 creatures is huge, and a swarm between 19 and 32 creatures is gargantuan.

Larger Hit Dice

Some high tier and epic level mobs might have standard hit dice of 2d6 or larger. For the purpose of these larger monsters, such as ogres or dragon wyrmlings, consider a grouping of hit dice as one hit die.

For example, a swarm of ogres could have 40 2d10+4 hit dice. The ogres only take damage in multiples of the average of 2d10s+4, or 15s.

A group of lower hit die creatures will have more total hit points. 80 d10+4 hit dice for a group of buff orcs would have a total max hit points of 800 and represents 80 creatures, up from 40.

For this reason, armies might only want one or two large creatures to soak in damage better than a swarm of them would. Many large creatures perform much poorer in swarms than they would as individuals monsters.

Mass Combat with Swarms

Swarm rules can be a great way to handle mass combats that feature heros and player characters.

These epic battles will often have two or more swarms of monsters in addition to the player characters. Allied swarms act exactly the same as enemy swarms do mechanically, and both can attack other swarms as normal.

Swarms can be easier to manage in larger scale confrontations by having them use all of their actions on a single turn in initiative. Swarms run by the DM can take their actions simultaneously when they act on the same initiative. All damage and other effects resolve affecting the other swarm resolve at the end of the current turn.

Major NPCs

In large scale combat, important NPCs can take actions that can bolster swarms and manipulate the battlefield the same way the players can. Sometimes an NPC will choose to hide in an allied swarm, lead the charge of his forces or hide back 400 feet manipulating the battlefield with spells and his archers while the grunts do the hand to hand combat.

In any of these cases, no rules need to be changed. A major NPC might be able to use new traits and features that directly interact with the swarms they command. A necromancer might have a spell that specifically heals swarms of undead by a large amount while a tactician may grant a swarm he shares a space with a flat bonus to all attack rolls they make using the swarm attack action.

NPCs with Legendary Actions could even have options to use some of their legendary actions to fuel a swarm attack or other swarm action!

Handling Weird Situations

Swarms present a challenging mechanical problem to 5th edition. Because of the simple nature of gameplay, many spells and effects could target a swarm that make no sense such as a fly and haste spell. Other situations like considering how hex, hunter's mark and bane function gets even weirder.

Rules as written, all of the above effects can affect a swarm as the swarm is considered to be a single creature.

Rules as intended allow spells like hex and hunter's mark to affect a swarm while disallowing spells like fly or levitate to affect a swarm. The intent is to allow players to feel epic while slaying monsters while steer them away from using the new swarm rules to attempt to make allied swarms affected by support spells like haste, fly and protection from good and evil.

Each DM should approach these rules bearing these intentions in mind and present the best solution for their table as necessary.

When playing using these swarm rules in two or more sided mass combats, consider using the rules as intended presented here to present a challenge to the players that fits in the realm of epic fantasy.