The Great Loss was an event that destroyed the entirety of the Arcadia Panopticon when they opened the object known as the "The Casket" killing everybody in a 10m radius.
While the circumstances of the event are unknown, its speculated that the Oltatum/Type-C class Abductor inside Simeon's dimension was liberated and lay waste on the Panopticon and its inhabitants, only to return to the casket after finishing.
According to a citizen, The infamous sinner Abel was rumored to be the only survivor of the event.