Chains of Obsidian (The Crystal Halls, Book Four)

Kuma and Pandora are on the run in the Eternal City—the deadly realm of the city fae. With her grandfather in complete control of the Undercity, Pandora knows their chances of defeating him are slim, but she's not willing to give up while she still holds breath. She leads Kuma on a dangerous journey through a realm filled with enemies to find a former mentor who she'd failed years ago. Hylakane—the Steel Sun—is an outcast, the last remaining member of Ebony Court and the only one she thinks might be able to help them. If she can't convince him to take her back as a student and unlock the potential everyone thinks she has, then everything she ever believed in will fall to ruin.