The Sacred Hunt Series by Michelle West

Once a year the Sacred Hunt must be called, in which the Hunter God's prey would be one of the Lords or his huntbrother. This was the Hunter's Oath, sworn to by each Lord and his huntbrother. It was the Oath taken by Gilliam of Elseth and the orphan boy Stephen--and the fulfillment of their Oath would prove the kind of destiny from which legends were made.

Now, at the risk of breaking their Oath, Hunter Gilliam and his huntbrother Stephen must journey to the ancient city once ruled by the Lord of Hell himself--a being who again seeks to impose his reign over the mortal lands. This novel continues the brilliant fantasy saga begun in Hunter's Oath.