Erinys

Snakes in the hair, a torch in one hand, and a very specific list. An Erinys pursues those who break the oldest oaths - kin-murder, guest-murder, the promise sworn on something that mattered - and it pursues them personally, without rest, across countries and years, and it does not become tired or bored or distracted. Everybody else present is furniture. It will step over them, walk around them, and not look at them once.

The Pursuit: it attacks its named quarry and no other creature, ever, unless physically restrained. Brazen Scourge: slashing, melee 10 ft, +5 to hit, 3d6+3, and a second strike with the torch, 2d8+4. Madness: 30 ft, quarry only, save Wis Mind, fail save roll 3d6+3 psychic, cooldown 2 rounds.

Not Your Fight: any creature interposing itself is moved aside, without damage, and it does not retaliate against them. A party can physically obstruct one for a long time at no risk at all, which is the only tactic that works and is also the one that requires them to decide whether they should. Cannot Fall: reduced to 0 HP it comes apart into smoke and snakes and is not there. It reforms at full HP beside its quarry at the next sunrise, wherever that quarry has got to, and resumes. Driving it off buys one night. Ends at Atonement: it stops permanently, mid-stride, the moment the debt is discharged - blood price paid, rite performed, the wronged party satisfied. That is the only ending there is.