Guardians of Ga'Hoole: #15 War of the Ember by Kathryn Lasky
The strange, maniacal blue owl known as the Striga has been ousted from the Great Ga'Hoole Tree. Nyra, leader of the vicious Pure Ones, is either dead or lying low in some distant land, leaving the tree finally at peace. As if fed by an invisible spring, learning and the lively arts flourish at the great tree and spread throughout the owl kingdoms. But unbeknownst to the Guardians, in a long-empty cave deep in the Northern Kingdoms two ruthless villains join forces to conjure up an ancient evil, an evil that will do their bidding and wreak havoc on the world.