The Casket is an ancient artifact made out of Will'O energy in the shape of a giant door. The Arcadia Panopticon was in possession of the Casket before triggering an event known as the Great Loss, caused by opening the Casket, and thereby wiping out Arcadia's entire population.
It was later found by the Protagonist's Panopticon. however, they were intercepted by Abel and were defeated, allowing him to escape with the Casket.
Abel took the Casket to the Hourai Panopticon and attempted to open it to bring the world into a unending war. However, he discovered that the Casket could only be opened with a specific Will'O signature. His spy (with the help of Julien Sadat, who was unaware of the spy) discovered that Beatrice's Will'O signatures could be used to open the Casket, and Abel immediately attacked the Protagonist's Panopticon and kidnapped Beatrice.
After using Beatrice to open the Casket, it liberated one of the Three Prototypes known as Oltatum, and the Protagonist and their team managed to defeat it. Shortly afterwards, a retribution is ordered on the Hourai Panopticon to destroy the Casket and its contents. Fortunately, the Protagonist and their team manage to escape the Panopticon in time.
It is revealed that the Casket is, in fact, a portal to a dimension habituated by a mysterious being known as Simeon, a god-like being with the ability to grant wishes to whoever he deems worthy.