Darkness within - Erin Hunter (Warriors: The broken code #4)

Goodreads 4.37

Exiled after a battle that left all five Clans devastated, ThunderClan’s loyal deputy, Squirrelflight, must bring to light an earth-shattering truth: the identity of the cat ruling ThunderClan while wearing Bramblestar’s face.

While the fate of his body hangs in the balance, the real Bramblestar’s spirit has vanished. Even Rootspring is powerless to reach him—and with tensions among the Clans at a breaking point, Squirrelflight may not be able to ensure Bramblestar has a body to return to. If he returns at all