The Shattering (Guardians of Ga'Hoole, #5)

Soren's sister, Eglantine, is falling under the spell of a strange nightly dream. Then, just as Soren notices her trancelike state, Eglantine disappears, and the dreams become a deadly waking nightmare that puts the Great Tree of Ga'Hoole in terrible danger. Soren must lead the Chaw of Chaws to rescue his sister. Thus begins the next battle between the owls of Ga'Hoole and the evil Pure Ones, deep in the treacherous territory known as The Beaks, where a raging forest fire will prove the greatest danger to the rescuers-and their best hope for victory.