Necromancy

Necromancy is an enchantment used to bring life to the dead.

There are two known instances of this, Nimueh brought the slain Tristan de Bois back to life as a zombie-like Wraith. It could not be slain by normal weapons, only enchanted ones, such as Excalibur. A wraith can only be created by using magic to harness the grief and rage of a tormented soul and returning them to a corpse like but indestructible body. No mortal blade can kill a wraith and magic is also useless against them for they are already dead. They are also said to disappear when their task is completed. (Excalibur)

Using a staff carved from a hawthorn tree on the Isle of the Blessed given to her by Morgause, Morgana was able to bring life to the dead of Camelot and use them to fight the Knights of Camelot as they fought the armies of Cenred in the Great Battle for Camelot. Although Morgana wasn't a experienced Witch the staff's power helped. The undead army were all skeletons and could be reduced to piles of bones, however these bones would continue to fight. After Merlin destroyed the staff, the undead army was reduced to dust. (The Tears of Uther Pendragon)