Dybbuk
About
A soul that could not go on because of something left undone, clinging to a living person and speaking through them in a voice that is recognisably somebody's. It is not a demon and treating it as one goes badly. It is a person, stuck, and frightened, and rude about it. Killing the host kills the host. The dybbuk steps across to the next nearest body before the first has finished falling, and it will keep doing that as long as there is anybody left in the room.
Stats
Save Rolls
Traits
Special Attack
Speak Through: it uses the host's body. Melee 5 ft, +4 to hit, 2d6+3, with whatever the host was carrying. Cross Over: on the host's death, or on any round it chooses, it moves to the nearest creature within 30 ft. Save Wis Mind or become the new host. Cooldown 1 round. It has always got somewhere to go.
Special Defense
Cannot Be Attacked Directly: all damage dealt goes to the host. There is no separate creature on the field and there never was. Naming the Unfinished Business: a creature that works out what the dybbuk left undone and states it aloud, correctly, ends the encounter permanently and the soul goes. The DN should decide this before play, seed it in the room, and let the dybbuk itself be the best source - it wants to be understood and it is terrible at saying so.