Does the damage from Thunderwave occur before or after the target is moved

Question

A PC is adjacent to a target which is a Magma Mephit. The mephit does damage to adjacent creatures upon death:

Death Burst: When the mephit dies, it explodes in a burst of lava. Each creature within 5 ft. of it must make a DC 11 Dexterity saving throw, taking 7 (2d6) fire damage on a failed save, or half as much damage on a successful one.

The PC has just cast the Thunderwave spell, targeting the mephit in the spell's area. This spell states:

Each creature in a 15-foot cube originating from you must make a Constitution saving throw. On a failed save, a creature takes 2d8 thunder damage and is pushed 10 feet away from you. ... In addition, unsecured objects⁠ that are completely within the area of effect are automatically pushed 10 feet away from you by the spell’s effect

The mephit failed the saving throw and took enough damage to kill it. The spell's description doesn't specify whether this would occur before it moved, or not.

The question Does Thunderwave's push effect knock away incoming spells/projectiles? is somewhat related, but the answer relies mostly in the instantaneous nature of the spells and the lack of the word "object" in the spells.

I think it's a reasonable DM judgment that the exploding bits of lava count as "unsecured objects" and that no damage would occur, but there are other impacts: another mephit is 10 feet behind it and if the explosion occurred later, that mephit would take damage from the explosion. Given that the mephits are concentrating on the Heat Metal spell (on the PC's weapon and armor), any damage requires a constitution saving throw which significantly impacts the PC's fighting ability.

Does its Death Burst do damage to the attacker, or was it pushed more than 5 feet away before its explosive demise, and may damage someone close to its new location?

Answer 1

Death at 0 HP for monsters is a shorthand

Creatures can make death saves at 0 HP optionally, but it is encouraged not to. Barring healing magic (which most monsters lack), if you are at 0 HP you are defeated - so D&D makes monster simply die.

If we assume any delay whatsoever between being KO'd at 0 HP and death, then the mephit will die after being pushed.

Xanathar says to defer to the creature whose turn it is

When there is more than one thing that should happen "at once", Xanathar gives the (optional) advice to defer it to the controller of the creature whose turn it is. Which in this case will probably be the thunderwave caster, who will probably want to push the creatures before they explode.

Forced movement doesn't need nerfing

In 5e, forced movement doesn't really do that much. Creatures are highly maneuverable in my experience, and being pushed a few feet doesn't change the tactical situation all that much usually. A spell that pushes an exploding foe away at the cost of being less effective otherwise shouldn't be treated with prejudice and made less useful.

So I'd let it push the mephits away, or position them, before their death burst occurs. This kind of spell (or ability) which is arguably the "best way" to handle exploding foes should be treated as if it can actually do that, and not ruled into uselessness.

In-fiction, faced with foes who explode when killed, a spell or ability that pushes them away makes perfect in-game-world sense to use. Making it not actually work because of a DM ruling seems like bad DMing - when a PC picks an appropriate tool to deal with a problem they are facing, saying "by my wording it does nothing useful" is not a good response.

Answer 2

This scenario presents several effects happening at the same time.

The Thunderwave spell does not provide any indication on the order of events, they seem to happen at the same time:

On a failed save, a creature takes 2d8 thunder damage and is pushed 10 feet away from you.

There are some features that list effects that happen in a particular order, using "then", such as Bigby's Hand (emphasis mine):

When you cast the spell and as a bonus action on your subsequent turns, you can move the hand up to 60 feet and then cause one of the following effects with it.

Xanathar's Guide to Everything provides some optional rule for such scenarios.

The Simultanous Effect section on XGTE provides some guidance on such cases:

Most effects in the game happen in succession, following an order set by the rules or the DM. In rare cases, effects can happen at the same time, especially at the start or end of a creature’s turn. If two or more things happen at the same time on a character or monster’s turn, the person at the game table — whether player or DM — who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character’s turn, the player decides which of the two effects happens first.

Hence, in this case, strictly following this optional rule, the Thunderwave's caster decide in which order the effects happen. Nonetheless, the DM has the final word on this, by what is referred to as "Rule 0" (DMG, page 4; emphasis mine):

The D&D rules help you and the other players have a good time, but the rules aren't in charge. You're the DM, and you are in charge of the game.

---

As a DM, since the Mephit's description tells us that

When the mephit dies, it explodes in a burst of lava

I would rule that the explosion happens as soon as it receives damage, i.e., when it is still close to the caster of Thunderwave and before begin pushed away, since the monster's death happen immediately when it reaches 0 HPs and in my mind at least a fraction of second passes before the corpse is moved 10 ft away from the caster. This means that the order I choose is

1. Receive damage
2. Death
3. Explosion
4. Pushing of the leftovers, if any

Answer 3

Save, then damage, then move.

The Mephit fails the roll and takes damage. It stops being a creature and dies before the movement of creatures comes around, but as an object it is pushed as an unsecured object. Spell operations done.

then boom

The special trigger of what happens with the Mephit corpse triggers after the spell is completely handled, so after the corpse is moved, as you don't splice separate effects into one another, you handle them after one another. This is not Magic where effects go Otto a FILO (first in last out) stack with interrupts, D&D handled effects in FIFO (first on first out).