Guardians Of Ga'Hoole #8: The Outcast book by Kathryn Lasky

To prove his goodness, young Nyroc will renounce his name, his mother, and his inheritance to seek the holiest of relics from The Beyond the Beyond. 

Nyroc has exiled himself from the Pure Ones. He flies alone, feared and despised by those who know him as Kludd's son, hunted by those whose despotism he has rejected, and haunted by ghostly creatures conjured by Nyra to lure him back to the Pure Ones. He yearns for a place he only half believes in -- the great tree -- and an uncle -- the near-mythic Soren -- who might be a true father to him. Yet he cannot approach the tree while the rumor of evil clings to him. To prove his worth, Nyroc will fly to The Beyond the Beyond seeking the legendary Relic and bring it, a talisman of his own