The War of the Ember (Guardians of Ga'Hoole Series #15)

The strange, maniacal blue owl known as the Striga has been rousted from the Great Ga'Hoole 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.