The Siege — Kathryn Lasky

TENSIONS ERUPT in the owl kingdom when the forces of evil wage war against the protectors of good. Enraged by his clash with Soren and driven by an all-consuming desire for power, Kludd and his group, the Pure Ones, launch an attack upon the Great Ga'Hoole Tree. The noble owls who live there must fight fiercely to protect their resources and defend their honor.

Meanwhile, Soren is called upon by the elders of the great tree to lead a mission back to the one place he thought he'd never see again -- St. Aegolius Academy for Orphaned Owls. He and his crew will have to enter St. Aggie's as spies, then leave unnoticed once their work is done. Soren escaped the rocky confines of St. Aggie's once. If peace is to be restored, he must do it again.