Wraiths

Wraith are the souls of dead clerics, returned to the world of living with magic. They are servant to the magician who had summoned them, and act in their name by turning other people to the way of evil. However, since this means being turned into an undead creature, it usually results in battle. Wraiths are very resistant.

It is impossible to say, where they can appear. Thankfully, with the defeat of Mondain, these unrestful spirits again went back to their graves.

Lore
''Summoned from the nethermost regions of Hell by Mondain's perverse enchantments, Wraiths are the restless souls of dead clerics. Once on the material plane, they drift about seeking potential converts to the discipline of Evil. Since one must first die in order to be converted, 'tis strongly advised that their enticements be resisted.''

- from "The First Age of Darkness", Ultima I