The Winter Long (October Daye #8) by Seanan McGuire
Synopsis from GoodReads:
For once, it seems like the Kingdom of the Mists has reached a point of, if not perfection, at least relative peace. Queen Arden Windermere is getting settled on her family’s throne; no one’s going to war with anyone else; it’s almost like everything is going to be okay. Even October “Toby” Daye is starting to relax her constant vigilance, allowing herself to think about the future, and what it might entail.
And then Simon Torquill comes back, and everything begins to fall apart. In Faerie, nothing stays buried forever. No matter how much you might want it to.