Bravelands 6 Oathkeeper by Erin Hunter

With the balance of the Bravelands at a breaking point, the Great Herd has found a common enemy to unite them: Titan . After eating the hearts of his victims, the rogue lion is powerful beyond measure, and even Fearless, who has vowed to avenge his father's death, cannot defeat him alone.

Thorn, the new Great Father, has a plan to bring about Titan's downfall. But the animals of the Great Herd must decide how much they are willing to risk--and who they are willing to lose.