DEATH'S DOOR
If you like, you can hold off on resolving this move until the scene wraps up or the other PCs get a little spotlight. If someone tries to save the dying PC (tending to wounds, etc.), then they're Aiding the Death's Door roll. The "risks, costs, or consequences" they expose themselves to will depend largely on how they Aid, your table's take on the Lady of Crows, and the result of the roll.
The Lady of Crows and the Last Door are for you and your table to describe. They might even appear differently for different characters. On a 10+, they're back in action and no longer dying, but they are marked in mind, body, or soul. The player decides how, but feel free to offer suggestions, like a nasty scar, a lost eye, visions of the Last Door, or a murder of crows, always nearby. Whatever they pick, bring the mark up often in play. Maybe even write it up as a threat (an affliction).
On a 7-9+, the character is no longer dying but they're unconscious (or close enough) until you say otherwise.
On a 6-, their time has come and the sane thing to do is pass through the Last Door. They can make one last move, but after that they're dead. There's no saving them. They can't use that last move to avoid death. Only the rarest of magic can bring them back.
If they refuse to go, ask why. Have them look at the Ghost or Revenant inserts and pick their Terrible Purpose. If they can't tell you, they shouldn't pick this option.
To become a Thrall, the PC must know the name of a Thing Below. If none of the Things Below have come up in play, maybe the PC can Know Things before dying?
If the character "sticks around," they are in for a steady downward progression, becoming more and more unstable and monstrous. Play up the horror. Show how their "lives" are no longer the same. Push their Terrible Purpose or Impulse into play. For a Ghost or Revenant, create their Terrible Purpose as an affliction; for a Thrall, create their master as a magical entity (see Threats). An undead PC should feel like they're on borrowed time—because they are. The other PCs should be wary of their undead ally. Wary, at the very least.