Gregor and the Curse of the Warmbloods (The Underland Chronicles 3)

Gregor and Boots must return to the Underland to help ward off a
plague. It is spreading fast, and when it claims one of Gregor's
family, he begins to truly understand his role in the Prophecy of
Blood. Gregor must summon all his power to end the biological warfare
that threatens the fate of every warmblooded creature.

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