The Siege (Guardians of Ga'hoole, Book 4)

by Kathryn Lasky

When an evil group tries to take control of the Great Ga'Hoole Tree, a fierce war breaks out in the owl kingdom. 

Soren's beloved mentor, Ezylryb, is finally back at the Great Ga'Hoole Tree. But all is not well. There's a war between good and evil in the owl kingdom. On one side is a group led by Soren's fearsome brother, Kludd, who wears a terrifying metal mask to cover his battle-scarred face. On the other side are the owls of the Great Ga'Hoole Tree, who must fight to protect their legendary home from Kludd's attacks.

Audio added Thanks to KundAlini