Egregious - Sanguine Blood Seekers, #5

Liam Lamech buried Valerian, the vampire he recognized as his mate, next to Hell's Gate nearly two thousand years ago. When the vampire king frees the vile creature, Liam determines not to give in to the strongest emotion a vampire can experience -- the pull of an eternal mate. Liam can't kill Valerian, yet he vows never to love him. Still, he'll protect the undead man with his own life. Valerian detests the memory of his days as a Roman emperor, though not nearly as much as he hates Liam, the vampire God chose as his mate.
Aware they were meant to spend eternity together, the monster buried Val so close to Satan that a piece of his soul still burns. Even though Liam is one of the vampire king's most revered warriors, Valerian vows to make the bastard pay!