Onryo

White robe, black hair, and a grievance that has outlasted everyone who could have settled it. An onryo is not hunting and is not hungry. It is owed something, specifically, by someone, and it will explain what if anyone stops screaming long enough to listen. It does not distinguish very carefully between the person who wronged it and the people standing near them, and by now most of the people it is owed by are dead.

The Grudge: 60 ft, save Wis Mind, fail save roll 2d8+3 psychic, cooldown 2 rounds, and it deals double damage to any creature that has knowingly benefited from the original wrong. Cold Hands: necrotic, melee 5 ft, +4 to hit, 2d8+3.

Scattered, Not Destroyed: weapons and spells hurt it normally and reducing it to 0 HP scatters it - but it reforms whole, at full HP, at the start of the next round. Damage buys exactly one round of respite and can never buy more than that, however much of it is dealt. Redress: it names what it wants, out loud, to anyone who asks, and it wants one specific thing - a body buried properly, a confession made publicly, a debt paid to a named family. Doing it ends the onryo permanently and instantly, mid-round, whatever its current HP.