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.
The World is actually a number of continents surrounded by oceans, which are in turn surrounded by mountains. The oceans rush off the edge of the plateau on which the World sits into the Outside...