Rising Darkness by Élianne Adams

He cannot die…
The phoenix will continuously rise from the ashes. For Zenon, this is his ultimate curse. Left to rot at the end of a chain for millennia, he is desperate to end the suffering of one horrendous demise, and rebirth, after another – to finally find peace. Desperate for anything other than his fated life of solitude. Desperate for his true death.

She cannot live…
Tormented by an unrelenting heart affliction, Nickie has to endure dying repeatedly. The physical pain of death is eclipsed only by the paralyzing fear of when her heart will stop beating next… and whether or not this time will be the last. That is, until she learns the myth of phoenix shifters, and the healing power they have over their human mates.

Together they could be everything…
An evil is rising. One more powerful than any the phoenix shifters have faced before. If Zenon and Nickie cannot convince the others to help, the pain they’ve already endured will pale in comparison to their future.