Revenants AKA the living dead  are not like the zombies of modern media. Most revenants fall under the category of Ghouls. Ghouls, or the Arabic word Ghul, are described as being demons in arabic culture ,but they are not demons in the christian sense. Ghouls are usually the cursed dead. They spend their time exhuming graves and eating dead bodies.

     In viking culture, ghouls were known as the Draugr which would haunt the graves of dead Vikings sometimes causing them to return from the dead and cause death and chaos in settled villages. In Eyrbyg Saga, a Draugr named Thorolf kills so many people that the entire valley it haunts is abandoned. Unless the Draugr's body was disposed of in some drastic way, it could come back. Like some accounts of the disposal of the bodies of revenants, the icelandic sagas suggest that the best way of getting rid of a draugr for good is to decapitate it, burn it, and then pour the ashes into the sea.

     Most revenants are created from a standard kind of Greek necromancy, although some of the more power practicioners added katabasis to their repertoires. In katabasis, a spirit projection of the necromancer travels to the underworld. A related practice known as katadesmoi, is one of the more dangerous things you can do. A spirit is summoned and the necromancer imposes a quest on it. As you can imagine, most spirits aren't thrilled with the idea.

     Death is the separation of the soul from the body. The creation of a zombie is the rebinding of body and soul via necromancy. The animated body can move, speak, even think, but it still can't outrun physical decay. Zombies don't last very long, and the more able they are to think, the more they suffer from the same derangement that eventually gets any spirit that's been prevented from moving on. It's a rule: if a spirit can't move on, the tug of the afterlife sooner or later drives them mad.