There are two different cases for a Vampire being in Mist Form.
- By volition: (Shapechanger ability)
- Via combat reduction to 0 hit points. (Misty Escape)
Shapechanger. If the vampire isn’t in sunlight or running water, it can use its action to polymorph into a Tiny bat or a Medium cloud of mist, or back into its true form.
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
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.
My concern is that the Vampire being at 0 in mist form may be an edge case for the general 0 HP further damage rule.
Here is my line of thought. The Vampire takes on mist form and stays at zero hit points. It is "in transit" in mist form to its lair at a movement rate of 20.
MM p. 297
While in mist form, the vampire can’t take any actions, speak, or manipulate objects. It is weightless, has a flying speed of 20 feet, can hover ... if air can pass through a space, the mist can do so without squeezing, and it can’t pass through water. It has advantage on Strength, Dexterity, and Constitution saving throws, and it is immune to all nonmagical damage, except the damage it takes from sunlight.
This leads me to believe that a party can destroy the Vampire without having to wait for two hours or put a stake into its heart when it arrives at its coffin/lair. The method is that the vampire in mist for can be destroyed by continuing to do magical damage until it fails 3 death saves.
Rule loophole: death saves may only be for characters, not monsters. It is unclear if monsters are utterly exempt from that provision, even though PHB 198 says.
the DM might have them fall unconscious and follow the same rules as player characters
As I read the description, the Vampire regeneration of 20 HP per round is constrained to its humanoid form, bat form, or it's misty form if it changed into that form by volition and while it has at least 1 HP.
A Cleric can typically move at 30, a Vampire's mist can move at 20. My instinct is that a party Cleric can cast Sacred Flame (Cantrip) round after round until a three death saves have been missed. Other sources of radiant or magical damage should work as well, as long at the damage being done is magical, to speed up the process.
- I chose Sacred Flame as it does radiant damage, which prevents regeneration in vampires, but that may be unnecessary.
- Paladin's Holy Smite
- Warlock's Eldritch Blast
- Rogue's magical dagger
Presuming that the Party keeps doing damage, and the Vampire misses enough death saves before it floats beyond their reach and through that crack in the wall that the party can't get through, am I right?
Is the Vampire Destroyed?
Damage at 0 Hit Points
If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer two failures instead. If the damage equals or exceeds your hit point maximum, you suffer instant death. (PHB, p. 197)
Does inflicting further damage on the misty 0 HP state allow the party to destroy the vampire?
Does the Vampire fail a death save as any other creature would, in the 0 HP misty state, when it takes any additional damage?
- Does the Vampire have an out that I don't see in the monster description or the rules?
A good answer will explain:
- Whether or not the Vampire is destroyed via this method
- If not, why not