Witchshadow (Witchlands 4)

At long last, Iseult’s story is revealed in the fourth novel in the New York Times bestselling Witchlands series, an epic fantasy adventure hailed by Alexandra Bracken (The Darkest Minds) as “a world you will want to inhabit forever.”
War has come to the Witchlands . . . and nothing will be the same again.
Iseult has found her heartsister Safi at last, but their reunion is brief. For Iseult to stay alive, she must flee Cartorra while Safi remains. And though Iseult has plans to save her friend, they will require her to summon magic more dangerous than anything she has ever faced before.
Meanwhile, the Bloodwitch Aeduan is beset by forces he cannot understand. And Vivia—rightful queen of Nubrevna—finds herself without a crown or a home.
As villains from legend reawaken across the Witchlands, only the mythical Cahr Awen can stop the gathering war. Iseult could embrace this power and heal the land, but first she must choose on which side of the shadows her destiny will lie.