The Slivers of Avalon: The Abandoned Edge

Seventeen-year-old Alexis has visionary dreams that take her to Avalon, the realm of faeries.
She discovers this other world is real, and it is where she was created. Switched at birth, Alexis is a changeling, simply disguised well to fit in with humans. Through a vision, she learns of her derailed heritage and about the day she was born, when the worst kind of evil killed her mother.

This evil is now after Alexis, and a prophecy insists she is the only one who can stop it. As she plans the creature's demise, it spends its time terrorizing both humans and faeries. To complicate matters, Alexis falls dangerously in love with a depraved faery. One who is close to the evil she is trying to stop. She feels connected to both worlds, and also to both dark and light. Struggling with who she is while deciding how to fulfill the prophetic claims, Alexis must choose a sacrifice to stop this abomination -- be it human, fae ... or even herself.