The black sands of the Three-Sands Desert are the home and final resting places of thousands of fallen beings called Zéví, those who have been claimed by the magical wards of the ebon granules, killed and brought back to life every night as reanimated corpses.  They are not zombies as they do not try to feed on the living; their job is to prevent others from crossing the black sand during the night.  If they get a hold of someone, they drag them beneath the surface of the sand, keeping them them until they are brought raised as another member of the dead army the following night.