Warriors (Erin Hunter). Original series

Fireheart, a full-fledged warrior cat, must confront questions of loyalty and identity as he faces the possibility of betrayal from within his own forest clan. Fireheart is a ThunderClan warrior now, but his troubles are far from over. The cats of the RiverClan are growing restless, while WindClan is weak and facing threats from all sides. Fireheart faces imminent battle-- but doesn't realize he also faces betrayal from within his own Clan. -- adapted...

Fireheart, the warrior cat, faces many challenges in his new role of ThunderClan deputy as his apprentice, Cloudpaw, resists following the warrior code, Bluestar weakens, and Tigerclaw continues to haunt the forest seeking revenge.

ThunderClan's darkest hour is upon them and Fireheart, the warrior cat, must protect his clan from a threat unlike any the forest has ever seen, as the time comes for prophecies to unfold and heroes to rise.