Runt the Brave (The Legends of Tira Nor, Book 1)

Driven by their blood-frenzied master, GoRec, a merciless army of rats swarms toward the underground mouse-city of Tira-Nor. The rats have come for only one reason: to slaughter the mice and seize their ancestral home. But rats are not the only nightmare to awaken in the dying days of summer. A cold and bodiless evil stalks the lonely tunnels of the city, a shadow whose presence may mean that the doom of the Ancients has risen once again.