Navecar

A navecar is the malevolent incorporeal Undead spirit of a Soul driven to insanity by forbidden or aberrative knowledge.
The name navecar is Elvish in origin, and is derived from the tale of Navec the Bryght, the scholar whose endless thirst for knowledge led eym to madness upon receiving an eye that could see as clearly as the goddess Morigana. Navec eymself was never a navecari, instead becoming the first Lich, but many of the devotees in eyr tower suffered this fate. Other names for the navecari include allip and wyrd-wraith.
A navecar loosely resembles the Humanoid shape of the person it once was, rendered in black fog. Its legs trail off into nothingness where it hovers, and its eyes flash wildly with pale, unearthly colors like will-o-wisps. In the Ethereal Plane, they appear to have writhing, grasping tentacles emerging from their heads, their mouths, and the ends of their hands. They babble incoherently in ciphers that include long-dead languages; any who try to understand their babbling risk becoming infected with their madness.
It is difficult to physically subdue a navecar in combat. They are far more vulnerable to magic, particularly magical radiance, than they are to damage from weapons. They attack by assailing their victims' minds with snippets of the forbidden knowledge that drove them insane and consuming the psychic energy resultant from that mental trauma. If not immediately resurrected, a person killed by a navecar will likely become a navecar themselves.
Navecari are likely to haunt any place that once held knowledge or magic long forgotten. The Cave of the Banished Scholars is said to have been infested with navecari, as well as the ruins of the Sanctum Aürum and the Black Keep. Fantastical stories of Dodiöu Réijmí told by outsiders often feature the navecari heavily.