Rabisu

A Rabisu is a Lower-Realm fiend that waits at doors, alley mouths, stairwells, and the moment between deciding to leave and actually stepping out. It is rarely seen clearly; witnesses describe hooked fingers, a crouched silhouette, and eyes level with the latch. The Rabisu attacks transition itself, choosing whoever crosses alone or last.

Threshold Ambush: slashing, melee 5 ft, +4 to hit, 2d6+2. The strike deals an additional 1d6 damage if the target crossed a doorway or boundary this round.

Cross Together: the Rabisu cannot use Threshold Ambush against creatures crossing side by side. Removing, opening, or clearly marking the threshold reduces its AC by 2.