David Farland 
Chaosbound 
Book 8 of The Runelords

The world of the Runelords is drastically changed. It has been magically combined with a parallel world, causing widespread destruction and chaos. Lands have sunk below the sea and elsewhere mountains have risen from it. Thousands have died, and humankind faces extinction.Borenson and Myrrima must journey to save their realm from total devastation, but the landscape is not all that has been altered. In the change, Borenson has merged with a monstrous creature from another world, Aaath Ulber. Not only is he transformed physically, but his behaviour also begins to evolve – he is becoming a berserker warrior. And as the dark rulers of this new world play a byzantine game of politics, this warrior is now the most significant pawn in that game.