Champion of the Twin Moons by Holly Bargo

In the enchanting world of the fae, Chastian, the gallant Champion of the Seelie Court, discovers his destined mate while visiting the Erlking—and she’s a human child.

Years pass, and Chastian eagerly awaits the day when he can claim his love and live happily ever after. But fate has other plans for them: the once-innocent child grows up and marries another, leaving him shattered and desolate.
The anguish of unrequited love proves too much for Chastian to bear, and he descends into a dark abyss of despair, consumed by sorrow and pain. His once-noble spirit becomes twisted and corrupted, turning him into a fearsome and ruthless villain.
Yet when the realm’s highest powers call upon Chastian to serve once more, he discovers a new purpose and a new mate—the daughter of his long lost love. Will she be the one to heal his wounded heart and restore his soul to its former glory?

Or will she reject him, like her mother before her, when she learns the extent of his corruption and malice?