Well, the players are all level 5 and I plan for them to have just hit 7 by the time they reach the BBEG. The killing blow upon the BBEG will break his artifact and open a rift between my world and a parallel world(Golarion). I want them to find themselves in a strange new world they have no previous knowledge of. So far the game has taken them through a great deal of sewers, some forest setting, and an abandoned elven temple that has melded to some extent with the planes of chaos. All they have left to do in this story arch is track the BBEG through some Morlock caves and fight a few Drow. It's my intention to leave my home grown world behind with no way to get back.