Demon Lord: Book I of the Demon Lord series

The Demon Lord will destroy the Overworld unless an innocent healer can turn him from his savage path... When the dark god Arkonen, imprisoned in the Underworld for an aeon, learns the secret of his release, he tears a boy child from his mortal mother's womb and raises him in the Underworld, where he suffers eighteen years of torturous training before the Black Lord sends him forth. Bane emerges through the world gate, and a dark army gathers to serve him. Thus, the Demon Lord sets out to conquer the Overworld and break the seven blue wards that bind Arkonen. Mirra, a young healer girl innocent of her destiny, is placed in Bane's path, and he takes her with him on his bloody rampage, defying Arkonen's order to slay her. Bane has never known kindness, and Mirra has not known cruelty, until now. He spurns her attempts to ease his suffering, unaware that the dark power he wields is killing him. Mirra must turn Bane from his dark purpose, or the Overworld is doomed...