Grisha: Ruin and Rising (book 3), The, Bardugo, Leigh

The Darkling rules the Grass from his shadow throne. Now the fate of the nation depends on the broken Sun Caster, the disgraced ranger and the broken remnants of the once great magical army. Deep in an ancient network of tunnels and caves, a weakened Alina must submit to the dubious protection of the Apparatus and the fanatics who worship her as a Saint. However, her plans are different: hunting the elusive firebird and hoping that the criminal prince will still survive. Alina will have to create new alliances and postpone old rivalries while she and Mel rush to find the last of Morozova's amplifiers. But as she begins to unravel the secrets of the Darkling, she reveals a past that will forever change her understanding of their connection and the power she possesses. The Firebird is the only thing standing between Ravka and destruction, and statements about it may cost Alina the very future she is fighting for.