All spell casters gain their abilities through a form of demon possession. So in order to gain magical abilites, PCs must track down and trap demons. Of course there are some pretty powerful demons, if the demon is stronger than the PC, the PC could find themselves subjugated to the Demon's will.
What a narrow street! The bowed windows of the upper floors encroach on the view of the bowed windows opposite, making it all very dark and shady down here in your carriage. You feel it slow down and stop, and there are raised voices outside. Craning your neck out of the door you see a smug cartsman ahead, whose cart is blocking just enough of the narrow street to make your passage impossible. He appears to be waiting for you to move, but your driver is hurling abuse at him and your horses are getting restless...