Chaosbound The Eighth Book of the Runelords

Borenson is set on a quest to save the devastated world from certain destruction. But the land has been altered forever: in the change, Borenson has merged with a mighty and monstrous creature from the other world, Aaath Ulber, and he begins to be a different person, a berserker warrior, as well as having a huge new body. Thousands have died, lands have sunk below the sea and elsewhere risen from it. And the supernatural rulers of the worlds are part of a universal evil, yet play a byzantine game of dark power politics among themselves. And Aaath Ulber is now the most significant pawn in that game.