Horsefaced Killer

From Introduction:

The entrance to the dungeon that lies beneath Saint Omen’s monastery is a closely-guarded secret, not least of which because those who use it don’t always come back. But those who disappear don’t always die, either. Or at least not the way most people do. Some become trapped, and some become transformed. Oigen Svenson was a mercenary thief, come to loot the dungeon of its treasures. He snuck in past the monks with ease, even though they were more diligent in those days. But in the dungeon, he met the nightmare and succumbed to it. Now he is a monster, ridden eternally by the demon horse that is trapped inside this inescapable prison, cursed never to die.