Demonhome (Champions of the Dawning Dragons #3) by Michael G. Manning

Matthew is the first human wizard to possess the true heritage of the Illeniels, a secret gift no one fully understands. Alone, he travels to another world, seeking the source of their mysterious enemies. There he will discover the origin of their ancient foe, the mysteries of the past, and possibly the future of humankind.

If he can survive long enough.

In a land beyond death and suffering, he finds the true source of evil, within the heart of humanity, and their newest creation. In the search for knowledge, some doors, once opened, can never be closed.