Gregor and the prophecy of Bane - Suzanne Collins

The second in a gripping young fantasy series from the author
of THE HUNGER GAMES, re-issued with a striking newly-designed
cover.

Spies have reported the sighting of a Rat King in
the Underland, a character who has been legendary since the Middle
Ages. Recognizable by its tremendous size and snow-white coat,
the Rat King is destined to bring war to the Underland.

But
when Gregor eventually comes face to face with the Rat King, he
finds he is unable to kill him. Gregor's heart tells him he's making
the right decision. Will it be one he lives to regret?