Corruption of Champions

From the introduction:

Centuries ago, a tribe of magic-blessed humans appeared in Mareth, building a home and eventual kingdom for themselves within the mountains. Talented at the arcane arts, they could manipulate the very fabric of reality itself with their magic, crafting whatever mundane item or object they so desired. Such a group would be peaceful, loving and happy with their lot, and indeed they might have been, were they not a large party of interdimensional explorers whose expedition became mired in Mareth. A quirk of Mareth’s nature prevented any known plane-shifting spell from transporting people off this world, instead forcing would-be travelers to rely on the many portals scattered across the world.