Gregor and the Prophecy of Bane - Suzanne Collins

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 a World War to the Underland. Gregor eventually comes face to face with the Rat King, and to his surprise, he finds he is unable to kill this creature. Gregor's heart tells him he's making the right decision. Will it be a decision he lives to regret?