A Contract?

The Titanomachy.

A ten-year war between the greek Titans and their children, the Olympians. 

It began after the children swallowed by the Titan Kronos were freed from the confines of their father's stomach, and declared their intention to establish a new rule. 

Out of all of mythology's most infamous and detrimental battles, the titanomoachy is largely seen as one of the worst and bloodiest; coming in second only to Lucifer's rebellion. 

In the end, two important events were necessary to bring about the gods' victory. 

The first was the creation of the three weapons of the swallowed gods; those being Zeus' lightning bolt, Poseidon's trident, and Hades' helm of invisibility. 

And the second was the freeing of their uncles, the hundred-handed-ones, from Tartarus.

Through these combined efforts, Kronos was sealed away, most of the titans were either pardoned or locked up with him, and the reign of the Greek Olympians began.