Wraiths

Wraiths are the spirits of dead clerics, which once haunted the dungeons of archaic Sosaria during the time of Ultima I

Description
Wraiths were returned to the material world from whatever underworld they inhabited by various enchantments set out by Mondain, and acted as proselytizing servitors of the dark mage, seeking to convert others to their debased state in worship of evil. It is unknown whether such wights were evil men before their death or if the possess by which they were conjured corrupted them, although references to their summoning from "hell" strongly implies the former - at least to those who hold to beliefs of a just afterlife.

These beings faded from the realm with the death of Mondain, likely no longer bound by whatever magics the wizard had employed to create them.