Darkmage: 1 (Rhenwars Saga)

When his own brother sacrifices the woman he loves to unseal the Well of Tears Darien finds himself the last surviving Sentinel capable of defending his world against the threat of the ancient Enemy. But Dariens hands are tied; he swore a vow never to use his power in the taking of a life. And now Darien is faced with an impossible decision: either remain steadfast to his oath and watch everything he loves fall around him or abandon his principles to become the most destructive force his world has ever known.