A Court of Splintered Harmony by Sarah J. Maas (Court of Thorns and Roses #6)

What was broken will be remade.

Deep in the heart of the continent, Koschei the Deathless is stirring. Though he's been bound to a mountain lake for millennia, the insidious threat of his return now spreads across the Fae territories, attracting powerful allies hell-bent on reclaiming what was once theirs: the human lands.

The Night Court will not stand idly by while innocents are attacked. But with Prythian still piecing itself together in the wake of the devastating Hybern War, the task of uniting is proving more difficult by the day.

To make matters worse, those who rule and protect the Night Court are reeling from recent betrayals and revealed truths. The rifts dividing them seem insurmountable.

But to defeat Koschei's creeping shadow, they must find a way back to each other and resurrect their former alliances. To survive, they must find a way to repair the splintered harmony all around them.