Citadel at Whitepeak
Magic in CoG is possible only through channeling the spirits of the dead, and, possibly, darker more terrifying things. All magic is dangerous, powerful, and difficult to control. Most sane civilizations severely punish unsanctioned uses of magic, and, at best, ostracize practitioners. Spells channeling benign spirits—principally worshiped ancestors and family guardian spirits—cast by highly trained members of priestly orders for the purpose of humble tasks such as healing and augury are accepted in most societies.
Black magic, magical practice that seeks to subdue the most powerful spirits in order to weave powerful rituals and spells, is forbidden in every human civilization except one, the Fion Order. Black magic adepts seek to bind the spirits of angry dead to their will. Black magic is able to accomplish far more than sanctioned uses. Its practitioners can command elemental forces, summon monsters, disfigure and even kill enemies through will alone (basically all the higher level spells in the PH).
In addition to being incredibly dangerous in its own right, black magic also has a tendency to corrupt those who use it. The angry spirits that fuel black magic loath being controlled. Thus, a black magic adept is engaged in a constant battle of will—attempting to control the spirit while the spirit attempts to control the adept. Adepts are haunted by the spirits they control. They scratch at the insides of their skulls, plague him with nightmares, sow hate and distrust in his thoughts, and even attempt to seize control of his body. It is a battle every adept eventually loses. Well-trained, strong-willed adepts can hold out for decades, but wielding black magic always leads to a slow degeneration in body and mind. Madness, paranoia, hatred, and physical decay and warping are the inevitable result. This is exacerbated by the fact that most adepts are driven to dominate evermore powerful forces in order to subdue those previous ones or to undue their degenerative effects. The weak and untrained can be consumed by a single ill-planned spell—immediately warped into foul monsters.