The Exile (Guardians of Ga'Hoole #14‪)‬

Sometimes the enemy comes from within...The Striga, former dragon owl from the Middle Kingdom beyond the Unnamed Sea, has come to stay at the great tree. He has earned the trust of all by saving Bell, Soren's owlet, from Nyra, and he grows daily closer to the young king Coryn, with whom he seems to share a strange bond. The Striga senses the power of the ember hidden in Bubo's forge and draws it closer. As his power waxes, he accuses the Band of treason and produces flimsy evidence to support his abominable claim. And so the Band is exiled, strengthening the Striga's hold over Coryn.