Gregor and the Code of Claw book by Suzanne Collins

The stunning conclusion to the riveting Gregor the Overlander series. Everyone in the Underland has been taking great pains to keep The Prophecy of Time from Gregor. Gregor knows it must say something awful but he never imagined just how it calls for the warrior's death. Now, with an army of rats approaching, and his mum and sister still in Regalia, Gregor the warrior must gather up his courage to help defend Regalia and get his family home safely. The entire existence of the Underland is in Gregor's hands, and time is running out. There is a code to be cracked, a mysterious new princess, Gregor's burgeoning dark side, and a war to end all wars.