The Darkest Hour (Warriors, #6)

ThunderClan's darkest hour is upon them, as Tigerstar's quest for power plunges all the Clans into the most terrible danger any cat has ever faced. In order to save his Clan and his friends, Fireheart must uncover the meaning of an ominous proclamation from StarClan:

"Four will become two. Lion and tiger will meet in battle, and blood will rule the forest."

The time has come for prophecies to unfold, and for heroes to rise ...