The “Bag of Rats” Problem is an issue that arises with RPG mechanics that trigger upon a kill. While kill trigger abilities are intended to reward the character for disposing of a foe, they incentivize players to metagame by carrying around a lot of easily-squishable pocket-sized creatures.

I was first confronted by this problem when creating my Psion homebrew. The Egoist subclass of the Psion has an ability called Enervate, which allows the Egoist to recover a Hit Die by leeching health from a grappled target:

The way this feature was originally written would enable the Egoist to heal a d8 Hit Die every round by expending their action (grapple rat) and bonus action (enervate). That’s probably not too overpowered in battle. However, the ability to instaheal using rats as potions after every encounter is likely gamebreaking. So how to we fix it?

Fixing the Problem

DM Fiat

Critical Role’s DM Matthew Mercer recently released his updated Gunslinger Fighter subclass, which attempted to address this issue by subjecting it to DM fiat:

The intended fix was to prevent the “bag of rats” exploit by ensuring that the defeated target is a “significant threat.” The parenthetical is explicit that this is purely at the discretion of the Dungeon Master.

This is a solution, but not a great one. While DMs and players should strive to achieve a balance of fair dealing, a rule which requires the DM to employ a subjective measure invites strife. It’s better to just have a clear rule.

The language is too squishy. “Significant threat” is not a term of art in 5th Edition Dungeons & Dragons. Therefore, what will be considered a significant threat can vary widely by the game, or even the battle. A goblin is a significant threat to a Level 1 party. Is it a significant threat to a Level 10 party? Not usually. What if the party is at the last of their resources and low on Hit Points? Well, then we might think it’s a threat. But what if it’s 1000 feet away and only equipped with a dagger? Probably not. But then, at what distance does the Goblin become a threat? Is the distance longer or shorter depending on how much HP and resources the party has? What if the party happens upon one Goblin guarding the entrance to a Goblin mine, but he’s fallen asleep on duty? The Goblin is not a significant threat while asleep, but would be if awoken. I would think that most DMs would be comfortable triggering the “on death” ability when the character kills a sleeping monster. However, that’s pretty clearly outside the scope of what constitutes a significant threat.

There are too many variables at play. You would need an entire set of rules to determine what creatures constitute a significant threat. When employing a mechanical trigger, its important to tie it to a phrase that’s already a defined term of art in 5th Edition Dungeons & Dragons.

Semi-Mechanical Solutions

Many spells and class features refer to “hostile” creatures. The Player’s Handbook (p. 185) briefly explains that hostile creatures are “inclined to get in your way.” The Dungeon Master’s Guide (p. 244) takes us further:

While that’s not exactly a hard-and-fast rule, it is employed in various spells and class features with success. The Aura of Life spell heals nonhostile creatures; the Light Domain Cleric’s Channel Divinity: Radiance of the Dawn feature targets hostile creatures.

Hard Mechanical Solutions

I recommend pulling from one of these hard mechanical solutions, when creating a spell or class feature with a kill trigger. It may vary depending on what the feature demands:

Size restriction (Small or larger) CR restriction (>0) Hit Dice (>1)

Size Limit

When revising the Psion, I fixed the problem by making the rats too big to carry. By adding a size restriction of Small or larger, I made the kill trigger reagent more unwieldy.

I thought this was a good solution for the Enervate feature, since the target should have enough life force to draw into the Egoist. It makes sense that larger creatures have a stronger life force. This actually plays out in the creature stats.

Hit Dice Limit

Drawing from the “life force” concept, I initially thought about limiting the ability so that you could only draw as many Hit Dice as the creature had. While I think that matches better, it requires a lot more bookkeeping and reference checking. It’s easy to remember that a Crocodile is large. If you forget, the information is visually available on the battle mat. If you don’t recall how many Hit Dice it has, you need to look it up.

CR Limit

Since only CR 0 creatures have one Hit Die, I was comfortable that the size restriction was also a fair approximation of the power of a creature. Some creatures are exceptions, such as the Giant Centipede (d6 HD, 1/4 CR), but it’s still a fair approximation.

Diseased Rats

You’re really carrying around that vermin?

Carrying around a bag of rats is not a risk-free proposition. Consider imposing other deterrents to metagaming.

Anyone trying to play the Pied Piper of Hamelin may suffer disadvantage in social interactions, as a result of their vile behavior. One may question how long the other party members will tolerate such behavior.

Small vermin are likely to carry diseases. Sticking your hand into a bag of rats is liable to get you bitten. Next thing you know, you might be dealing with a nasty case of Sewer Plague (Dungeon Master’s Guide p. 257).