Moonrise (Warriors: The New Prophecy #2) by Erin Hunter

DARKNESS, AIR, WATER, AND SKY WILL SHAKE THE FOREST TO ITS ROOTS.

Moons have passed since six cats set out on a journey to save their Clans. Meanwhile, back in the forest, Firestar and Leafpaw watch ThunderClan's world crumbling around them. Will the questing cats make it back in time, or will they be too late?