One of my players placed their eldritch cannon inside the mouth of an enemy. How should I handle the attack?

Question

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?"

Answer 1

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:

DM: Okay, it's your turn. What do you do?
Player: I shoot that guy with my cannon. But first, I put it right in his mouth.

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:

They managed to place their weapon inside the mouth of the enemy with a touch attack roll

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".

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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.

Answer 2

From the PHB (p6):

1. The DM describes the environment […]
2. The players describe what they want to do. […] the DM listens to every player and decides how to resolve those actions.
3. The DM narrates the results of the adventurers' actions.

In other words, just because a player says that they (want to) put the gun in the creature's mouth it doesn't mean that it happens, let alone confer bonuses to other actions - the same way that the character saying "I jump over the chasm" will require a skill check.

Therefore, it's up to you to decide how to resolve the action "stick the gun into the mouth" and there are three basic ways:

1. Decide that this is trivial, eg the creature is unconscious or paralyzed, so it happens automatically
2. Decide that it's impossible, the same way it's impossible for a character to say jump to the moon, no matter how high they roll on Athletics
3. Decide that there's a chance of this happening and requesting a suitable roll

Then you get to decide what are the implications (and that could be a way to balance the challenge). For example, if we end up considering it an autohit, it makes sense to request that they make an opposing athletics/acrobatics check (similar to grappling), effectively replacing the attack roll with the check. For an auto-crit, I'd consider that it's the equivalent of two successful hits in one round - so perhaps something like two skill checks or a skill check with disadvantage (to cover the two eschewed attack rolls) plus triggering an attack of opportunity from the mob. I wouldn't go as far as auto-kill (although if the mob is left with 1hp you could rule it dead); you can explain the creature surviving in various ways, from "their brain wasn't there in the first place" to "as you fired, they tilted the head and the shot went through their cheeks".

I'd highlight that this is an one-off ruling and if the character wants to make frequent use of this, they should find a similar maneuver/spell/feat. Frankly, I feel that the entire sequence is more fitting as the description of what happens after a critical hit is scored, as oppose to a way to guarantee crits.

Answer 3

There are no hidden rules. The cannon works as normal.

In general, in DnD 5e, there are no hidden rules. The rules in the book are what they say they are. If the player shoves the end of their Eldritch Cannon into the mouth of an enemy before pulling the trigger, they'd make a ranged spell attack or force a Dexterity Saving Throw as normal, depending on the option chosen for the cannon - and if it's the former option, the attack roll would even have disadvantage since they're in melee range of an enemy, unless they have one of the feats that removes that penalty.

If you want to justify it in-universe, presumably, on a miss or successful save, the enemy managed to pull the barrel of the Eldritch Cannon out of their mouth.

Answer 4

You shouldn't have allowed the placement in the first place

The rules for placement of an Eldritch cannon are finite:

At 3rd level, you learn how to create a magical cannon. Using woodcarver's tools or smith's tools, you can take an action to magically create a Small or Tiny eldritch cannon in an unoccupied space on a horizontal surface within 5 feet of you. A Small eldritch cannon occupies its space, and a Tiny one can be held in one hand.

As there are no hidden rules, nothing allows an Eldritch Cannon to be placed inside of another creature. You even have to create it in an unoccupied space in the first place. In fact, achieving such positioning is precluded in the first place, as you can only move through other creature's space, but never end your movement in the same space:

Whether a creature is a friend or an enemy, you can’t willingly end your move in its space. - PHB p191

Even the Grappling maneuver does not make two characters share one space but puts them into adjacent ones.

Since you can't put the cannon in the mouth (that would be willingly ending in the square) in the first place, this should technically moot the question.

Only if the enemy would try to swallow the cannon or otherwise ingest it with some feature it could end up in the mouth under the rules.

Ok, you did it, what now?

Well, for some reason or another it happened anyway. How to rule now? Well, the closest similarity to what the cannon in the mouth reflects is the Shambling Mound's engulf feature:

Engulf: The shambling mound engulfs a Medium or smaller creature Grappled by it. The engulfed target is Blinded, Restrained, and unable to breathe, and it must succeed on a DC 14 Constitution saving throw at the start of each of the mound's turns or take 13 (2d8 + 4) bludgeoning damage. If the mound moves, the engulfed target moves with it. The mound can have only one creature engulfed at a time.

That's... BAD: Treating the cannon in the mouth as Engulfed means it is blinded, restrained, takes bludgeon damage and also grappled. Luckily being Grappled don't technically preclude a ranged attack (Grappled condition only puts the speed to 0), but Restrained gives automatic Disadvantage on the Attack of the cannon, and then there is Disadvantage from shooting point-blank... which luckily only accounts for simple Disadvantage, because it doesn't stack.

You might feel inclined to grant a simple Advantage for the good position to counter the Disadvantage from being technically Grappled and Point Blank, resulting in a normal Attack Roll.

Conclusion

So the ruling of the cannon placement should have been: "You can't do that, only the enemy can try to gulp down your gun."

In case you allowed it anyway (or it happened on the volition of the enemy), the ruling now should be, because of the conditions that should apply (restrained) and that automatically apply (point blank ranged attack), that there has to be an attack roll with Disadvantage or, if you are particularly mercyful, a normal one.

Answer 5

The DM holds ultimate power but...

So, before I get into how I have adjudicated such instances before, I'll start off with the proper disclaimer. It is entirely up to you how this will be ultimately adjudicated. While we can provide some anecdotal evidence to support answers, there's not going to be anything better than "What you think is best". Simply put, you are the god of your world and should you decide then that is the decision that flies. However.

Mechanics vs. Narrative

Mechanically, if you allow something spectacular then it should have an imposed risk. The rule of cool only goes so far in creating amazing DnD experiences. The dice can make better stories than any author because there's always a risk. So to, getting rid of dice can make an entire moment less sweet. Of course, there are always exceptions.

So let's say you just allow this enemy to be slain instantly, no save. Then there should be a narrative/mechanical bridge to cross. Is the enemy important? Is there a reasonable chance they might survive a shot to the mouth (DnD monsters have strange biology)? Is the ruling that you create something you're willing to enforce down the line (players will remember this ruling for the rest of the campaign so think carefully). Then make the decision as necessary.

Ultimately, there are rules that can support your interpretation or at least Variant Rules. Massive Damage is a good way to add extra wiggle room if the shot would deal 50%+ of the target's max health. But I would caution against going any further. A max health creature taking a good shot but not being pushed past half their health is hardy for a reason. Unless...

Narrative vs. Mechanics

If you are a certain kind of DM, then the Mechanics mean less than the narrative they create. It can mean that you want this type of home-rule in your game where the players get to feel even more powerful. Or it can mean that you accidentally create a situation in which all your Artificer does is try to get in close and head-pop every enemy.

In such circumstances, I recommend still doing a similar mechanical/narrative bridge. But look at it from the other end. Throw away enemies are meant to be discarded. An enemy, outside of combat, could be in such a bad situation that the dice would ruin the situation by making it take way too long. Balancing mechanics vs. narrative flow is something every DM approaches differently and it may be in your favor to let this one pass.

Ultimately, your decision is based on whether you prefer the narrative flow of doing such an action or whether the dice support it. I caution toward mechanical answers and the rules of the arcane cannon are pretty specific about what it was designed for. However, that might not work for you and it should always be encouraged to have some level of creative freedom in a DnD game.

Anecdotal Evidence

There are several instances I can bring up of both sides of the coin on insta-kill status. One of the more prominent in my mind is an old Rollplay show on Dark Heresy. The squad has a distraction go into a bar, the (would-be) assassin sneaks in through the back, puts their shotgun up to the back of the guy, and shoots him... in the leg. Because that's where the dice said he got shot. And it didn't even do enough damage to kill. Some ganger from a hive city took a 12 gauge to the back and lived (briefly) to tell the tale.

On the other hand, back in my early days of DnD our party (not knowing at all what we were doing) somehow snuck into the BBEG chamber and had our rogue do a series of extremely unlikely things (without rolling) and slit the BBEG's throat. And it was cool in the moment, but it's very hazy as time goes on. I can't even remember more than that.