You've made a pact with a supernatural entity. You get supernatural powers, and it gets... what? Surely the blood sacrifice wasn't all it wanted. What impossibly higher order game is being played here? But honestly, do you really care? Because you're a witch now.
A witch is much like a Mystic in many ways. They are both granted miraculous powers by a supernatural entity in return for whatever that entity desires. Most mystics trade prayer and proselytizing for their powers, while others act on behalf of their patron entity to impact the course of events. Witches are also asked to do these things, but so much more. In return for being granted even greater powers than mystics, witches must also make correspondingly greater offerings. First is usually a blood sacrifice of some sort to demonstrate conviction and to provide the power necessary to seal the contract. Furthermore, a witch allows himself or herself to become an actual conduit of the entity into this world. They invite the entity to share their body in something called the gift of union. This results in several different effects. First, it transforms the witch's body into a supernatural body. Being supernatural, a witch could be impervious to most normal weapons, immune to poisons, resistant to most magic, or might even be able to fly. The rewards provided match the nature of the pact. Second, the witch gains an inherent ability to cast magic of the kind known by the entity. This could be psionic, magic, technowizard or miraculous, or some other form of power not yet known to Rifts Earth. Third, the witch gets a familiar. After proving himself or herself worthy and faithful, the witch is gifted with a familiar. This familiar grows out of the witch and leaves behind a nipple. Thereafter the familiar gains its sustenance from that nipple. It is wondered whether or not witches retain their free will, for it has been noted that they typically have an immediate change in personality. Let it be known that witches do retain their full faculties of free will. However, they do usually abandon their inhibitions either due to the liberation of so much power or due to the insipid whisperings in their minds of a being infinitely more intelligent than they.
As a novice, the witch manifests one supernatural ability and is automatically granted an arcane backround, regardless of education or history. These are the gifts of union. A seasoned witch manifests a second supernatural ability and a familiar springs forth from the witch's body. A veteran witch manifests a third supernatural ability and the ability to summon beings from the entity's dimension. An heroic witch manifests a fourth supernatural ability and enters into a major pact, making the witch ageless in return for his or her eternal soul. A legendary witch manifests a fifth supernatural ability. Any other powers and abilities of such powerful witches are completely unknown.
A witch never needs to learn spells or abilities. Such things are granted to them by their patron entity. They just know them. They may still go about learning skills and training for abilities, but this is rarely done because reliance of the witch wholly upon the gifts of the supernatural entity serves the entity much better. Since a witch's body becomes supernatural, they are immune to the effects of Ley Line Sickness.