Redeem — C.S. Kendall

Finally, after a century apart, Julia and Charlie begin building a life together. But once again, their plans for a future are thwarted when Charlie displays puzzling side effects of his supernatural existence as the Phoenix. As his symptoms progress and Julia runs out of options, they are forced to seek out the only other immortal they know-Charlie's former captor, Amara.Darius and Amara go back millennia. Having drunk from the murderous Fountain of Youth, their history is marred by deadly choices. Amara is desperate to have Darius back but he cannot forgive her for the part she played in unleashing the Fountain of Youth on the world or for what the water made her do.When Julia gets wind of Amara's insidious plan to redeem herself, she finds that an enemy of an enemy is a friend. After saving her life, Darius reveals the truth about Charlie's existence as the Phoenix, including the fact that this time, Julia can't save the only man she's ever loved. Armed with nothing but a shadowy foretelling about the Phoenix, Julia discovers that with Charlie and his newfound abilities they may be able to stop Amara from unleashing her treachery on the world. The only problem is they have no idea how. Even if they could figure it out, will the cost of bringing Amara down be too high?