Warlock

A Warlock is an individual whose magical power is dependent on the whims of another specific entity, with whom they have entered a pact. A Warlock is a practitioner of magic, but their powers are mostly determined by this external entity. Warlocks do not always get a choice in the matter, and are often merely puppets of greater powers. The greater powers in question are able to, at a whim, remove the powers from their warlocks.
There are many different entities which can act as the patron for a warlock's pact. Most often they are Demonlords, or Ancient Dragons, however there have been cases of warlocks forming pacts with Archangels, Fey creatures, powerful Spirits, Ghosts, or even Arcane entities. In some cases, the warlock patron is an item, such as a sentient weapon, which binds itself to the wielder.