So the vulnudaemon is—
No, vulnu. With an N.
Stop snickering. I’ll wait. I’m the son of an OB/GYN; you cannot embarrass me.
So the vulnudaemon, based solely on its illustration, looks like your standard possessed and/or zombified creepy child. (“Aw look, her blade’s dripping blood and everything.”) Then you look more closely…and you realize that slit in her neck isn’t the wound that killed her. It’s a mouth. You can just picture a cleric seeing her in the streets as she stumbles from the doorway…him running up to her to apply his healing touch before the life pours out her jugular…only at the last minute to see that slit widen into a toothy grin…and seeing only too late the knife coming down to end him.
That is the vulnudaemon in a nutshell: murder spiced with betrayal from the body of a child.
And while they recall the vengeful spirits that spawn certain undead, the soul that forms a vulnudaemon is not going to stop after dispatching her killers, her family, or other innocent children like herself. The vengeful dead have rules, categories, taboos to obey. Daemons defy all of the above, because daemons are the end of all of the above.
Following their lord’s orders, adventurers guide his wife and child to the city of Mersk. Upon arrival, the mother turns around and sells the child into slavery. (Unbeknownst to her husband, she has arranged a more advantageous match in Mersk and refuses to be weighed down by former attachments.) Then she tries to have the adventurers killed. Meanwhile, the child dies at the hands of his new master, and a particularly vengeful vulnudaemon inhabits the corpse.
Fenton Chesterfield has kept a vulnudaemon bound in his tower for months, probing it for secrets of the dark planes. The bonds are beginning to slip, though—the daemon’s aura of doom, even just felt through the floor, has managed to drive Chesterfield’s maid into a deep depression. Meanwhile, cacodaemons have begun to lurk near the base of Chesterfield’s tower, preying on stray dogs as they build up strength. If not intercepted, the shaken maid will soon either commit suicide or kill her master—pleasing the daemon either way.
The doppelgangers who mind the child beggars of Punjar have turned to the worship of daemons, and now at least three vulnudaemons walk the streets wearing the skins of betrayed children. They murder other beggars (both children and adults), local priests who provide services to the unfortunate, and any alms-givers traveling too late at night. For adventurers, cleaning out the daemons and the doppelgangers is only half the trouble—if they do not find a strong leader to look out for the beggars’ interests, a tribe of skum are poised to move into the sewers to pick up where the doppelgangers left off.
—Pathfinder Bestiary 3 63
A teensy bit more on vulnudaemons (including their true forms and their role on the plane of Abaddon) can be found in Todd Stewart’s Horsemen of the Apocalypse.