As with many legendary artifacts, the Book of Eternity has its origins in Greek mythology. Cronos, the celestial offspring of Gaia (Mother Earth) and Uranus (Father Sky), ruled over the Titans and the entire world in this tale. Cronos was imprisoned in Tartarus by Zeus and the other Olympians after he was vanquished by them in the Titanomachy.
Nonetheless, there is more to the story of Cronos's confinement than meets the eye. Even while Zeus and the Olympians wanted to keep Cronos imprisoned in Tartarus, they understood that this would not be enough to restrain his great power. They made the bold move of locking Cronos and all of his influence away in an ancient tome they dubbed the Book of Eternity.
It was said that the Book of Eternity held the secrets of the universe and had the power of eternity itself. The book's cover included an Ouroboros, an ancient symbol of a serpent or dragon that eats its own tail, with Cronos's sign superimposed on top.
The three most potent Olympians—Zeus, Poseidon, and Hades—were entrusted with safeguarding the Book of Eternity. They came to the conclusion that the book should be kept hidden in the mortal realm, where it would be safe from harm as long as they both lived long lives. Furthermore, they swore to never reveal the book's existence to anybody, not even their fellow Olympians.
The Big Three's other crucial mission was to safeguard the book from would-be thieves and other people who could abuse its influence. The immortals were aware that there were those among them who would do anything to free Cronos and seize his authority. In order to be ready for such a scenario, they constructed magical devices named the EternalProject and endowed them with some of their own power. For the purpose of safeguarding the book's contents, these objects would be bestowed upon specially selected mortals.
Therefore, Zeus, Poseidon, Hades, and their chosen mortals kept the Book of Eternity concealed and protected for eons. But the search for the truth about the book and the power it holds has never fully ceased; even today, there are individuals who would strive to unlock the power of eternity by discovering the mysteries of the Book of Eternity.