The Dormant Calamity, Thrasos

This being is said to be the slayer of the gods and Primordials, as well as the bringer of the end. The Overgod Ao created this behemoth to protect the gods' realms from the Primordials, in case they crossed the boundaries separating their domain's. When this Behemoth began to defy his creator and threatened to destroy the realm of the gods and usher in an era of calamity, Ao imprisoned him. This prison was created in the mind of the Behemoth, to keep it in an everlasting state of slumber beneath Mount Celestia.
Other names; The Dormant Calamity, The Chained Extinction, Gods Bane.