The Rescue (Guardians of Ga'hoole, Book 3)

Now that Soren has been reunited with his sister, Eglantine, he must face his next challenge: making sense of the mysterious disappearance of his mentor, Ezylryb. When Soren discovers that Ezylryb is in danger, he and his friends Gylfie, Twilight, and Digger devise a plan to save the beloved teacher.
In this process, Soren must fight a ferocious foe who wears a terrifying metal beak, sharpened for battle. It's not until the confrontation is over that Soren discovers the true identity of his opponent. The metal beaked warrior is Soren's evil brother, Kludd.