Along with his fellow Titan Phoebe, Coeus was a Titan of the Moon. Both were the offsprin of Gaea the Earth and Uranus the heavens, and along with their fellow Titans of Myth they were god-like beings.
However, Gaea's other children were horrible monsters and Uranus banished the brood to the pits of Tartarus. Gaea persuaded Cronus to kill Uranus and free the brood - he completed the first part of his task, and then invited Coeus and the rest of the Titans of Myth to join him in a paradise of subjugating the Earth.
Many years later, Cronus' son Zeus released his siblings and the brood from the pits and Coeus and the rest of the Titans of Myth were imprisoned in columns of stone in Tartarus.
Thousands of years later, Thia and Hyperion managed to release themselves, and with the help of the Teen Titan Wonder Girl (Donna Troy) they freed Coeus and the others as well. Coeus now joined the other Titans of Myth in an assualt on the gods of Olympus, and after two epic battles in which some of the Titans of Myth were killed, Zeus invited the remaining members to join him in living in peace inside Olympus. Coeus and the others accepted.
Later, Coeus and the other New Titans took up residence on New Chronus. Donna Troy, killed and reborn, was his wife. The New Titans, led by Hyperion, were waging war against Minosyss a planet where a great weapon was hidden. It was a complete slaughter, the warriors of the New Titans massacring the natives.
One of Donna Troy's adoptive siblings called out for the Teen Titans, sending a god gate to bring them to Minosyss. The Teen Titans sided with the Minosyns, trying to make Donna remember them. Donna did, though she feigned ignorance for a longer…
The weapon Minosyss hid was a sun eater, capable of rending the space between dimensions. Donna opened a gate, pretending it would lead the New Titans to a universe they could shape to their wills. Instead she trapped them in Tartarus. Coeus knowingly went through the gate, nobly swearing to keep the rest of the New Titans trapped there.