Gregor and the code of claw - 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 awful: 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.