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.
As they travel they notice a horrible rotten smell. As they travel it gets stronger and stronger forcing them to cover their noses and mouth less they choke on the horrible smell. Rotting animal carcass that has been ripped apart and tracks all over, covered in maggots.