This is the 6th book of the Warriors series. It was written by Kate Cary.
About the book
FIRE ALONE CAN SAVE OUR CLAN... ThunderClan's darkest hour is upon them, as Tigerstar's quest for power plunges all the Clan 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 procla- mation 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. . . .