I remember reading somewhere that tiny creatures can't kill players/bring them down to 0 hp in normal conditions, which makes sense to me. A rat biting someone's toe and dealing that 1 damage would be ridiculous to die to. However, I can't recall where that rule is in the various books. Can someone tell me if it exists or if I'm making it up? As well, if this rule is true, does this apply to greater damage sizes? Say, if a wasp dealt 11 poison damage? Does that actually harm a player?
There is no such rule.
I have over 1500 D&D 5e posts on this site and have read the PHB and DMG cover to cover numerous times, as well as every other supplement at least once. I am confident there is no such general rule, or any rule at all relating a creature’s size to its ability to kill player characters.
As an easy counter example, the CR 18 demilich is a Tiny creature.
It is worth noting that there is a meme within the community that a cat is a deadly encounter for a commoner, which would not be true if this rule existed.
Michael Richardson, in his answer, recalls an article written by Sean K. Reynolds (one of the contributing authors of D&D 3e) that mentions this problem, and in this comment, David Coffron traces the article via way back machine. The article describes a house rule called “The Phil Athans Rule” that may be the source of your recollection:
The Phil Athans Rule
If a creature with a Strength penalty to damage makes an attack, and the damage rolled would be adjusted to 0 or because of the creature's Strength penalty if it weren't for the "1 point of damage minimum" rule, the creature's attack deals 1 point of subdual damage instead of normal damage. This rule does not apply if the creature takes a -4 penalty to its attack rolls (just as if it were trying to deal normal damage with a subdual-damage weapon). (If you're using the 3.5 rules, replace "subdual" with "nonlethal" and "lethal" with "normal.")
There is no such rule ...
See Thomas Markov's answer for the assessment of a site veteran much more accomplished than myself. That being said, although I'm not quite as accomplished, I can only support his conclusion.
... and it wouldn't make sense, either
It's easy to think that it would make sense, if all you consider is 1 rat vs 1 human. However, unlikely as that is, a rat could certainly kill you if it bit you in the jugular, for example. The same is even more true for cats, which have very sharp claws. Similarly, I would most certainly not want to fight a hundred rats, because if the rats actually try their best to kill me, I don't think my chances are particularly good.
That being said, more importantly, a rat would almost never choose to fight a human if left a choice. Because yes, it's obviously at a huge disadvantage (in real life), even though that's not perfectly represented in D&D 5e - which isn't exactly designed for pathetic 1v1 human vs rat fights, and hence doesn't model this edge case very well.
Most importantly, however, there are some extremely dangerous animals that would have a size of Tiny in D&D 5e terms. This includes animals like Jellyfish, scorpions, bugs that transmit diseases (like Mosquitos), snakes, and a variety of other poisonous/venomous animals like some frogs, fish, or spiders.
Granted, they generally use poison/venom to kill you, and some of them would take quite a long time to kill you (perhaps long after they themselves died to you), but they can kill you nonetheless.
And that's just the animals that we know from real life. If you include harmless tiny fantasy creatures like, say, Demiliches into the equation, it should be obvious that tiny does NOT equal benign.
If it bothers you that small, non-toxic animals can kill humans, you have two solutions (which you can also combine):
- Just don't make them fight humans. That's the most straightforward/realistic approach, because unless backed in a corner, small animals will generally not want to fight humans, and if backed into a corner, even small animals can be dangerous.
- Just don't use their "official" stat blocks. The Monster Manual may have a stat block for a rat that allows it to potentially kill a human, but nothing forces you to use that stat block. You could just create your own custom stat block that includes a constraint that it can't deal damage to creatures more than 1 size larger than itself, for example.
I wouldn't recommend a general rule that "tiny animals can't kill players", because again, there are absolutely tiny animals that can do just that.
And honestly: is the problem really going to come up frequently enough to warrant a proper house rule, instead of just being implemented with a custom stat block or two?
Back in v3.5 there was an article on Sean K Reynolds site (seankreynolds.com, now defunct?).
This article made reference to the deadly cat referenced in Thomas Markov's answer. I believe he attributed the anecdote to a conversation among developers, which included Phil Atkins (may not be quite right, one of the v3 and/or v3.5 developers).
I believe Sean came up with a rule published on his site, "Atkins Rule" I believe he called it, which converted the damage done by tiny creatures that are dealing a single point of damage, from lethal to subdual.
This would prevent a housecat from going on a rampage and slaughtering a village.
It has been more than a decade since I read it, so the mechanics may not be exactly as I remember. I bring it up as it was a direct mention of the deadly cat by one of the game developers and his anecdote to having an internal conversation about it with other developers.
Logically, Small creatures can kill players.
- Rats will inflict disease.
- Bugs can inflame players' skin
- Bacteria are the source of disease and a lot more things.
- Mold can eat and cripple flesh
However little the creatures may be, generally in my humble opinion, the smaller the creature gets, the more it it should be that it kill outright, and the more it should relies on the status effects it places to still be considered a threat.
If the DnD system that you are using does not fit your needs, I'm of the opinion that revising the stats of certain tiny creatures to fit your play is the way to go, although it may need some tuning to be introduced into a cherished and continuing session without throwing the story off course or breaking the playability entirely.
As you can probably tell, I tinkered with my current worlds quite a bit without hesitation.
Edit: As KorvinStarmast had mentioned in the comments, A rot-grub in DnD5e is one such creature. It is a parasite that lives in corpses and functions as follows https://www.5esrd.com/gamemastering/hazards/rot-grub/
Generally, a handful of the grubs infest a single corpse at a time. A DC 10 Wisdom (Perception) check is enough to notice and avoid the grubs. If the grubs go unnoticed, contact with the corpse results in 1d6 grubs bursting from the corpse and beginning to burrow into the creature’s flesh. The creature must succeed on a DC 12 Dexterity saving throw or take 1 piercing damage from each grub and be infested with rot grubs.
Within 4 hours, the grubs will have started to burrow toward the host’s heart, brain, and other internal organs, eventually killing the host.