Warlock

Despite the many dangers, some Witches survive without submitting themselves to the judgement of the Imperial Colleges and in time, grow in experience and strength. Using Dark Magic is immensely dangerous, even more so than using one of the refracted Colours of Magic. There is little training a Human can undergo to defend against the corrupting and physically destructive affects of using it. Yet some such Witches grow to become ever more aware of the nature of the Dark Magic they are using, and through sheer force of will and an ego of borderline or true megalomania, they even manage to find ways to harness it with increasing dependability, apparently wrestling it into a semblance of submission.

Essentially, Warlocks are successful and powerful Witches who are very aware that they are using Dark Magic, but do not care—in fact they rely upon it. Unlike all Hedge Wizards or even most Witches, these few are no longer innocent bystanders, dupes, or ignorant fumblers with powers they do not understand. They are fully aware of what it is they are doing and actively enjoy the incredible raw power at their fingertips. Whether through trial and error, supernatural assistance, through the rare instruction of another Warlock, very rarely, an illegal secret society, or through finding or stealing tomes of forbidden knowledge, Warlocks actively seek to study the ways of Dark Magic and endeavour to become proficient in cruel and destructive spells to further their own ends.

By merit of the fact they use Dark Magic so frequently, Warlocks pose a very real danger, both to those around them and to themselves. They have towering egos, iron-like will, and are always very hardy physically. Because they knowingly and willingly seek out and have embraced Dark Magic, they also tend to be paranoid megalomaniacs.

Few Warlocks are alike in terms of talent. As they are generally self-taught, they develop individualized spells and abilities. However, all Warlocks tend to fall in one of two groups: Daemonologists and Necromancers.

Source

 * Warhammer Fantasy RPG 2nd ED -- Realms of Sorcery (pg. 129).