Gregor and the Prophecy of Bane (The Underland Chronicles 2)

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?