The War of the Ember (Guardians of Ga'hoole, Book 15)

The final book in Laskys best-selling animal series. The strange maniacal blue owl known as the Striga has been rousted from the Great GaHoole Tree. Nyra leader of the vicious Pure Ones is either dead or laying 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 unbeknowst to the Guardians in a long-empty cave deep in the Northern Kingdoms two ruthless villains join forces to conjure an ancient evil an evil that will do their bidding and wreak havoc on the world.