Warlock

A mage that siphons off the essence of souls and uses them to cast spells.
A warlock learns spells quite differently than other mages, unlike wizards or sorcerers a warlock does not study a spell to understand it before they can manifest it, they steal the knowledge of how to cast the spell that resides within a mage's or spirit's soul. The process does not need to be fatal, although it often is, but it is destructive as the victim will lose the knowledge of how to cast the spell in question as well as a small fraction of themselves, such as memories. These fragments of the victim are embedded within the warlock, often overtime driving them to madness.