The Demon Prince

This story follow Illyria, a member of the Goddess Clan. During the Holy War, the Goddess is mortally wounded, eventually causing her death. As she was dying, she was saved by a Vampire.
She believed that it was Zeldris that took her life on the battlefield, but when that is proven to be wrong, her whole world changes.
Why is it that the youngest Demon Prince has such a hold over her, even after he's locked away in the Coffin of Eternal Darkness? And what will happen when the seal is broken?