Wraith

A wraith is a slain person reanimated by dark magic, seeking revenge for past events.

Properties
They cannot be slain by normal weapons, only enchanted ones, such as Excalibur, can destroy them. They are also said to disappear when their task is completed.

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.

History
The knight Tristan de Bois, slain in a duel with Uther, was reanimated by Nimueh, seeking revenge on the king, but then finally destroyed when struck by Excalibur in a second duel against Uther (Excalibur).

Allusions
A wraith is similar to the Inferi of the Harry Potterverse. They are also similar to zombies, being reanimated by a Dark Wizard, similar to Nimueh's reanimation of de Bois.