Gregor and the Curse of the Warmbloods (Underland Chronicles Series #3)

Gregor is summoned back to the Underland by the terms of a second prophecy. Spies have reported the sighting of a Rat King, 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. Is Gregor right to follow his heart?