Grave-walkers. When the most sadistic, depraved villains are buried, sometimes Orcus curses their spirt to rise again as an abhartach. The withered corpse rises from its coffin, and fills its worm-eaten husk with grave dirt. They stalk the lonely roads, graveyards and catacombs, drawing power from the dead and seeking living creatures to slay and consume.

Cursed wanderers.  Though these undead hunger to destroy and consume living creatures, a shard of memory remains. Abhartachs recognize cherished items, old friends and adversaries from their former life and never pass up an opportunity to destroy them. Once they've destroyed everything and everyone tied to their old life, they wander aimlessly, drawn to places where the dead are kept such as churchyards and cemeteries. 

Eternal suffering. An abhartach can only be killed if it is stabbed with a sword or knife made from rowan or yew wood and then buried upside down in the roots of the same tree. If not, the abhartach will eventually return and will seek out those who harmed it.