Tethered Wings (Hidden Wings Series Book Three);

Back from their quest into the Underworld, with the threat of Lucian hanging over head, Emma is faced with yet another problem—the Immortal Bond. The bond surpasses any reason and is something so complex, not much is known about it. Once an Immortal Bond is made, it is forever. A conjoining of two fated hearts, destined to be perfect companions.
Emma’s heart, already bonded with her Guardian, Kade, has unexpectedly and inconveniently bonded with Ethon, the son of Lucifer. Now she is faced with a decision she doesn't want to make. Both want nothing more than to love and protect her.
With the arrival of new company and old friends, things are bound to get heated.