Percy Jackson: Flames of War

Sequel to Spark of Betrayal.

As the two camps come together under dire circumstances, older powers begin to rise. With the absence of the gods and the ancient magics that protected the demigods, there is nowhere left to hide. As they fight for their lives, they realize they aren't fighting for their own, but the very existence of the Fifth Age. But they can't survive and win alone.

FULL CREDIT TO Tears of a Spirit. on fanfiction.net