Source: The Fall of the Titans by Cornelis Corneliszoon van Haarlem.
Author's Note: This origin story is an odd one. Cronus, the leader of the Titans, eats all the children he has with Rhea, a fellow Titan, out of fear. Rhea, with the help of Gaia, yet another Titan, saves her youngest son Zeus from her husband. Zeus was hid away on Mount Ida on Crete with the Kouretes. Zeus eventually confronts his father and makes him expel the swallowed children. Zeus also releases the Hecatoncheires and the Cyclopes from Tartarus. I also reference the Titanomachy as it is a major part of the Olympian God's story. This was a 10 year war in which the Olympian Gods, with help from the Hecatoncheires and Cyclopes, defeated the Titans. After this war, the three brothers drew lots for the realms of the earth (sky, earth, and underworld). You can learn more about these stories through the links listed in the bibliography (Wikipedia really allows you to go down so many holes... don't get too lost, haha).
Instead of sticking to this same story, I decided to throw in a little twist in younger Zeus' story line. I never specifically referenced the Kouretes, but instead had a Cyclopes be his friend in hiding. The Cyclopes' are responsible for Zeus' lightning abilities, so I thought it would be fun to have him develop his power in young adulthood instead of during the Titanomachy. I also threw in my own fictional details on how some things occurred (like the swapping of Zeus for the rock, the expelling of the children, etc.) which you can see if you read the original iterations of this story. Another goal was to establish the dichotomous sense of personality between Zeus and Hades, but also show how they can unite and accomplish things together. Finally, I wanted to stay true to Hades' voice and his promises in the introduction.
Bibliography: Hades on Wikipedia, Titanomachy on Wikipedia, Zeus on Wikipedia, and Hades on Theoi