[Mithadan]

Sauron's alias before the war of the Ring was the Necromancer, meaning one who enchants the dead. HoME (Morgoth's Ring I think) states that when Elves die, they are summoned to Mandos, but that Morgoth (possibly through Sauron) made a counter-summons seeking to capture Elven spirits. These were presumably put to use by Morgoth (and later Sauron). To some extent, Mannish spirits might also be subject to capture. The Dead of Dunharrow evidence this possibility though that was not Sauron's work.

After receiving their Rings, the Men who later became Ringwraiths became sorcerors. Some of their powers and instruction likely came from Sauron, the Necromancer. Thus it is not surprising that the Witch King possessed necromantic skills. The Barrow Wights were probably dead bodies re-animated by the spirits of captured Elves or Men captured or co-opted by Sauron or his servants.