The Cursed King by Abigail Owen

Airk Azdajah, the rightful King of the White Clan of dragon shifters, spent half a millennium being tortured by the false High King Pytheios. The only reason he's alive is a curse: the man who kills Airk will be consumed in his own fire. So Airk was kept alive—barely—and unable to shift in his prison cage, driving the creature half of him into madness. Airk escaped, but he'll never be truly free. Because what good is a king who can never let his feral dragon loose, never fly, and never lead his people?

Angelika Amon is the last unmated phoenix. The problem? She has no powers. Zip. Zilch. Nada. Angelika hates being dormant, especially now that her three sisters are blissfully mated to powerful dragon shifter kings and are very much part of the fight to take down the rotting king Pytheios. What good is a completely powerless phoenix in a battle to save the dragon kingdom?

Desperate to find some way to help, she offers herself to Airk as a mate—just for political leverage. Only Airk would never permit Angelika to tie herself to a dragon who can't shift. But that might just change when Pytheios decides to claim Angelika for himself...