They are powerful undead, the sort that have a vague sense of who they used to be, but can no longer remember themselves. They are embroiled in bitterness so great that they lash out upon the living, using the abilities they had in life as a weapon of their own torment.

[He collects one of his journals from the chest, flipping through the pages until he comes across a drawing of an emaciated figure, its fingers and limbs writhing, face contorted in a perpetual howl. Its eyes glow vibrantly with an unnatural light against its ruined frame, barely illuminating its twisted visage.]

Depending upon the body from which it is created, they can be tremendous warriors... or even magicians of utter desecration. Surely only a the most powerful of necromancers could raise such a being. That is what I've always been told. That's what everyone is told.