Cephissus

Cephissus was once a demigod child of Apollo, but was turned into a sea monster for turning against his father. Cephissus held a grudge against his father and the Olympians.

The Last Olympian
Cephissus was mentioned in The Last Olympian where he was serving Kronos to destroy the Olympians. Before he could succeed, he was defeated by Talos and sent his ashes back to Tartarus where he could reform for the next thousand years.