Alas, there isn’t much to say about vampiric mist, which is what you end up with when the body of a vampire is destroyed but its essence isn’t. With no way to form a new body, it floats around aimlessly, feeding off victims by employing a sort of necrotic vacuum effect to pull blood out of victims’ pores and facial orifices.
I normally begin by looking at a creature’s ability contour, but in this case, there’s not much point. There’s only one stat that matters, and that’s its Intelligence, which is subsapient. Vampiric mist has no judgment, only instinct. Moreover, it has no attack action per se, only Life Drain, an effect that requires a saving throw to resist. Vampiric mist isn’t so much a creature as it is a punishment.
Because of their Sunlight Sensitivity, vampiric mists come outside only after dark, and they don’t mess around with civil, nautical or astronomical twilight. It’s nighttime or nothing.
Also, thanks to the Forbiddance feature, one is safe from vampiric mist as long as one is inside a residential building, either one’s own home or someone else’s. (“The mist can’t enter a residence without an invitation from one of the occupants,” but seriously, who’s going to invite a grayish-crimson, foul-smelling cloud of vapor to come inside, especially one that can’t even knock on the door or answer the question, “Who’s there?”) Inns are a gray area: If you’ve ever read a zoning ordinance, you know that inns are commercial, not residential. On the other hand, a rented room at an inn can be an individual’s primary place of residence and therefore, in a legal sense, their home. As Dungeon Master, you make the call regarding whether a player character has a permanent enough arrangement with an inn to construe their room as a residence. There’s no ambiguity around monasteries and convents: as both permanent residences and hallowed ground, they’re safe. But adventurers spend a lot of time on the road, and a tent is not a building, period.
Anyway, vampiric mists are just out for blood, and they don’t care where they get it. If they can’t breach the perimeter of your boudoir, they’ll zero in on some shlub out for a midnight stroll instead. Battlefields, full of bleeding wounded and impermanent bivouacs, are magnets for vampiric mists.
Once you’ve decided you’re going to throw a vampiric mist at a group of characters who’ve left themselves vulnerable, there are no decisions of any consequence to make. Vampiric mists don’t Dodge, Disengage, Hide or do anything like that. They Life Drain, again and again, until they’re defeated. They have no independent judgment and no ability to communicate. Their above-average Wisdom gives them some ability to assess targets, but it doesn’t extend far beyond picking on the already weakened and shying away when struck by a magic weapon.
Unlike a garden-variety predator, however, vampiric mists don’t flee when they’re wounded. As undead, they’re driven by compulsion—in this case, the compulsion to feed. Vampires, the regular walking-around kind, are smart; while they, too, are compelled to feed, they have enough enlightened self-interest to plan out how they’re going to do that at minimal risk to themselves. Vampiric mists have no such intelligence, only the compulsion. Once they’ve started feeding, they don’t stop until they’re destroyed. Hurt them with damage they aren’t resistant to, and they don’t waft away—they only move to the opposite side of the battlefield. They aren’t smart enough to know that opportunity attacks are a thing. Only one thing actually drives them away: sunlight, either natural or from a sunbeam spell.
Next: sword wraiths.