Mages work their spells through sheer strength of will, forcing the magic out of the ether and into the physical world, this causes a backlash that causes the mage great pain, sometimes even physical harm, eventually ending is either the death of the mage, or gibbering madness as their mind snaps from the strain.
there are two ways to alleviate this, one: a network of mages joins their power together and shares the backlash among themselves, this is the most common method.
the second is rather more dark. in the ether there are demons, the twisted remnants of dead mages. a living mage can offer his body as a host to a demon, giving him great power, as the demon has a greater connection to the ether, which alleviates much of the backlash...this comes with two major consequences, however. the first is that the mages body, unable to handle the shee energy coursing through it, begins to take on changes, becoming increasingly obvious depending on the power of the demon, ranging from subtle changes in skin tone, to their entire body twisting into new and horrifying shapes..the latter being, of course, excruciatingly painful.
the second price is that the demons force compulsions onto their hosts. sometimes it can be something as small as OCD like tendencies, such as having to dress in a certain order, or repeat a certain action..to horrible acts..torture, cannibalism, rape, and all other wanton acts the twisted mind can come up with.
A mage can become host to as many demons as he can find, eventually becoming twisted, violent abominations..but capable of changing the world with a single thought