Absorb a spirit and take its energy
Spirit Absorption is the ability to absorb an the essence of spirits and use their life force as a source of energy or nutrition.
The only known user of this ability was the Necromancer. As the spirit of a demon, he required the energy of spirits to sustain himself and become corporeal. The length of time for which he remained corporeal depended on how strong the absorbed spirit was. For example, the spirit of a Fairy sustained him for a longer period of time than the spirit of a troll. The Necromancer required a great amount of magic, like that of the entire Warren line, to be resurrected.