Dark Warrior Mine (The Children of the Gods, #7)

When Andrew is forced to retire from active duty, he believes that all he has to look forward to is a boring desk job. His glory days in special ops are over. But as it turns out, his thrill ride has just begun. Andrew discovers not only that immortals exist and have been manipulating global affairs since antiquity, but that he and his sister are rare possessors of the immortal genes. Problem is, Andrew might be too old to attempt the activation process. His sister, who is fourteen years his junior, barely makes it through the transition, so the odds of him coming out of it alive, let alone immortal, are slim. But fate may force his hand. Helping a friend find his long-lost daughter, Andrew finds a woman who's worth taking the risk for. Nathalie might be a Dormant, but the only way to find out for sure requires fangs and venom.