As stated in the title, one of my players has put their eldritch cannon (Artillerist Artificer) inside the mouth of one of the enemies in their current encounter. I'm struggling to figure out how to handle this situation.
They managed to place their weapon inside the mouth of the enemy with a touch attack roll, and succeeded, which makes me angle towards wanting to give them some form of reward for this action.
Older editions suggest something akin to a coup-de-grâce strike, instantly killing the creature because of the situation that it's in. I want to be able to reward the player for planning this out, but I also don't want to set an expectation that they should be able to one-hit any enemy simply by placing their weapon in their mouth.
I can't find rules for coup-de-grâce strikes in 5e, which makes this a little more problematic, along with whether or not this should be considered a coup-de-grâce to begin with, which makes me think that it should instead simply be an auto-critical, but there is still a part of me that wonders "should it kill the enemy outright instead?"
Best Answer
If there were actually unusual circumstances that let your player do that with his cannon, then the unusual circumstances should also be granting attack bonuses. For example, if a creature is "unconscious" or "paralyzed" then any attack that hits the creature is an automatic critical (if within five feet). If your player was able to put the cannon in the creature's mouth because it was unconscious or paralyzed (or some similar status effect) then giving your player a free critical is 100% fair.
On the other hand, if the conversation went like this:
then you can't give him a benefit for that. If you do, then every attack for the rest of the adventure he's going to be saying "I put the cannon in his mouth first" and getting guaranteed critical hits or whatever.
In particular, you've written:
and from context I think you mean that you allowed this roll as a bonus action or less (else, the enemy would have moved on its turn). I get that you want to reward player creativity, but "spend a bonus action to get a chance to upgrade your attack to an auto-crit" is way broken and you need to not set this precedent.
Instead, you should be saying: "the creature is moving around and keeping its mouth closed and you have no way to put your cannon in its mouth".
In general I advise against upgrading any attack to instant kill. Even a strike against a paralyzed or unconscious target is only an auto-crit. If you let them do instant kills then they will find a way to do it to the final boss of your campaign and you'll wish you hadn't allowed it.