Shadow's Light

"This shall be a difficult task, child. There are Fae people out there who want to kill you for what you may become."

In a world where dark and light faeries must coexist, Rhoslyn (a lowly light faery) lives and serves inside the royal castle. When she accidentally discovers a secret, too terrible to speak of, she finds herself and Áedán on a journey into the human world, where no other faeries has gone in over 600 years. Rhoslyn must unravel the secrets that grips her family and Avalon's before it's too late.

Áedán, a dark faery, and the king's trusty guard. A man of seductive charm and dark power whose harbouring a dangerous secret, Rhoslyn can't stand to be near him, but there's this invisible energy luring her to him.

Thanks to @creator15 for the help with the summary

Please give this story a chance. It does get better.