[Keeper of Dol Guldur]

The wight Frodo and the hobbits encountered was said maybe to be the last prince of Cardolan-in body, but a good necromancer like the witch-king, since he was part of the physical and spiritual world may have been able to "grab" a spirit as it left the body. The term "fell spirits from Angmar" which came and inhabited these places implies maybe dead dwarves, since Carn Dum could have been an ancient dwarven fortress corrupted by one of the seven rings. Maybe the witch-king or Sauron when they ruined this place, put up a barrier and trapped the spirits within so they could corrupt them over time. Indeed Sauron was a master of fell spirits-like the werewolves, which he bred and inhabited with "fell spirits", and the nazgul which he turned into wraithes. My guess is that through the teachings of Aule he learned how to build a hall for spirits, not unlike but lesser to the halls of Mandos, and trapped spirits for his uses.