Warriors (Erin Hunter). Omen of the stars

As Dovepaw, Jayfeather, and Lionblaze strive to understand and fulfill the prophecy that promises them great power, the cats of the Dark Forest begin to teach battle skills and techniques to different Clan cats through dreams.

The end of the stars draws near.
Three must become four to battle
the darkness that lasts forever. . . .

The dark forces that have driven a rift between the four warrior Clans are growing stronger. Jayfeather, Lionblaze, and Dovepaw now know that unless they can unravel the true meaning behind the prophecy that binds them, the warrior code could be destroyed forever.

While Lionblaze remains focused on protecting ThunderClan

...

"After countless moons of treachery, Tigerstar's Dark Forest apprentices are ready to lay siege upon the warrior Clans. As Jayfeather, Dovewing, and Lionblaze prepare to lead their Clanmates into battle, they await the arrival of the mysterious fourth warrior who is prophesied to help lead the Clans to glory." Publisher's description.