rules question

[FabulousFizban]

Misty Escape. When it drops to 0 hit points outside its resting place, the vampire transforms into a cloud of mist (as in the Shapechanger trait) instead of falling unconscious, provided that it isn’t in sunlight or running water. If it can’t transform, it is destroyed.

While it has 0 hit points in mist form, it can’t revert to its vampire form, and it must reach its resting place within 2 hours or be destroyed. Once in its resting place, it reverts to its vampire form. It is then paralyzed until it regains at least 1 hit point. After spending 1 hour in its resting place with 0 hit points, it regains 1 hit point.

what happens if i hit the mist with moonbeam, is the vamp destroyed? (moonbeam says shapechangers revert to their base form if hit by it)

[Amnestic]

what happens if i hit the mist with moonbeam, is the vamp destroyed?

Depends on your DM.

If the vampire's got death saves, moonbeam would tag it for one fail, as on any other creature with death saves.
If it doesn't, then yes, it would kill it.

The mist has immunity to nonmagical damage, but magic weapons or spells work just fine.

[RSP]

I’d probably say yes, vamp is destroyed due to reverting to its humanoid form, while not also triggering turning back into mist (it’s still at 0 HPs in mist form so it isn’t hitting 0 again).

[stoutstien]

DM call. Id say moonbeam triggers a death save seeing how they are generally mid tier npc or low level BBEG

[Kurt Kurageous]

Absolutely DM dependent.

From DM view, any full vampire that gets into that situation without doing a bat-dash escape at half HP deserves to die. Any full vampire that allows a party to put so much DPR on clearly wants to be destroyed. Feed and leave is the standard approach, preferably from someone charmed so they don't have to grapple.

But slugging it out to 0 HP? Deathwish or bad play by the DM.

[Mastikator]

I agree with DM's call but as a DM I'd say destroyed vampire. Vampires are pretty weak for CR 13 if they don't play to their strengths and slugging it out is the opposite of playing to their strengths.

[Segev]

By the RAW, the vampire's specific text overrides moonbeam's general text about forcing shapeshifters to revert: the vampire cannot revert to its vampire form, so moonbeam can't force it to, any more than a dominate spell could force a goblin to sprout wings and fly. In this case, it takes an automatic failed death save from moonbeam's damage, remains at 0 hp and in mist form, and continues to do whatever it wants to do (but probably trying to get to its resting place without being followed).

This is a little bit of a DM judgment call, though: I argue that the vampire's text is the more specific rule, but I could see somebody making the converse argument that moonbeam's forced reversion is more specific. I think it an incorrect argument, but close enough that I could see somebody disagreeing with me.

[NecessaryWeevil]

I would say you end up with an unconscious, solid vampire. Who, yes, fails a death save due to Moonbeam's damage.

Reasoning:
Vamp hits zero HP and is not currently under the effects of Moonbeam. Vamp turns into mist.
Vamp is hit by Moonbeam and reverts to solid form, as per the spell. "Incapable of reverting" in the vampire's text means, to me, "Incapable of reverting via its own abilities." Segev is correct, above, that Dominate can't make a goblin sprout wings and fly. But Fly can (well, aside from the wings). The inability to do something natively does not prevent a character from being enabled/forced to do something, by the appropriate magic.
Solid vampire is unable to transform into mist, but it is not instantly destroyed because "if they are unable to transform, they are destroyed" only triggers at the moment they hit zero HP.

PS On the other hand, instant destruction in this scenario is nicely cinematic and rewards the PCs for applying the right tool to the right situation. Which is what vampire hunting tends to be all about, in the fiction.

[Chronos]

I'd say that the line in the vampire "If it can't transform, it is destroyed" covers the interaction well enough. Certainly, if it was the Moonbeam itself that reduced the vampire to 0, then it would be destroyed, because the Moonbeam prevented it from transforming. In fact, the Moonbeam spell is probably the exact reason why the vampire rules include that line, because I'm not sure what else there is that could prevent a shapechanger from changing shape. One could argue that there's a difference between trying to turn into mist while in a Moonbeam and being hit with the Moonbeam while already mist, but I think that the intent is clear.

Also, even if the vampire is an important NPC, they don't get death saves. Like most undead and constructs, they don't even die: They're instead destroyed when they hit 0 HP, and that happens regardless of the rules for death.

Which, yeah, does make Moonbeam an extremely effective spell for vampire-hunting: It gives them disadvantage on their saves, as shapeshifters; it does repeated radiant damage every round, to stop their regeneration; and it prevents them from using any of their shape-changing abilities, either voluntarily or on hitting 0. As NecessaryWeevil says, vampire-hunting is all about using the right tool for the job.

[NecessaryWeevil]

Also, even if the vampire is an important NPC, they don't get death saves. Like most undead and constructs, they don't even die: They're instead destroyed when they hit 0 HP, and that happens regardless of the rules for death.

Ah, good point.

[Witty Username]

I am mostly with moonbeam smites on this one. I think it is unclear if a misted vampire can be reverted after being reduced to 0, but moonbeam would have killed it before that.

I looks like the intention would be the vampire gets dusted.

Note though, that is definitely worth spending legendary resistance on, so expect that to still be a ordeal some of the time.

[Chronos]

Note though, that is definitely worth spending legendary resistance on, so expect that to still be a ordeal some of the time.

Sure, but it's still a second-level spell slot that can force a save every turn, and which it's quite likely to fail without the legendary saves. If you're going to need to burn through legendary saves, then that's a very efficient way to do it.

[Witty Username]

Sure, but it's still a second-level spell slot that can force a save every turn, and which it's quite likely to fail without the legendary saves. If you're going to need to burn through legendary saves, then that's a very efficient way to do it.

Yes, but moonbeam is only 2d10 damage on a creature with almost 150 hitpoints, so that will not always draw out legendary resistance (I would expect that to be in the case, that the vampire either can't move out of the moonbeam, or it has already lost a number of hitpoints, as those are the times it would need misty escape, other wise it will take the damage), I suspect at least some of the time, the vampire would simply leave the moonbeam and either attempt to run, break the casters concentration, or offset the damage with its bite attack. Heck if it is already grappling someone I could see them trying to do something cheeky like drag them into the moonbeam to mess with the party.

That and I was thinking of the situation of a vampire already at 0, getting hit with a fresh moonbeam, which depending on how the combat was may still have all of its resistance.

Moonbeam is a great spell for this for a number of reasons, I just don't think burning legendary resistance is very high on that list.

Oh, this is all assuming the Vampire is not fighting in bat form, I don't think that will be very often though. Maybe a vampire spellcaster that is using the web spell (homebrew list territory)