A spectre is an undead creature created through a combination of wickedness and a violent death. They are ghostly, resembling what they were in life, but possessed of a revulsion for the living, and a hunger for their life-energy. In addition to inflicting level drain, specrtres create spawn like themselves when they kill.
A spectre resembles the person it was in life, at the moment of death. It is transparent and faintly luminous.
Spectres first appeared in OD&D's white box, where they are described as totally incorporeal, and where their level-drain is especially potent.