Tartarus

In Greek mythology, Tartarus (/ˈtɑːrtərəs/; Ancient Greek: Τάρταρος, romanized: Tártaros) is the deep abyss that is used as a dungeon of torment and suffering for the wicked and as the prison for the Titans. Tartarus is the place where, according to Plato's Gorgias (c. 400 BC), souls are judged after death and where the wicked received divine punishment. In Hesiod's Theogony, Tartarus is described as one of the earliest beings to exist, alongside Chaos, Gaia (Earth) and Eros (Love).
In Greek mythology, Tartarus is both a deity and a place in the underworld.
However in world, this being has never physically manifested in a form, as it would kill all the players instantly. But, Oto did get drawn into an entrance to Tartarus and fell to it's depths, drawn there by it's resident Titans. Here Oto came to terms with their being whilst trying to escape from this place, unfortunately he was unsuccessful and succumbed to it's denizens.
An entrance to the pit alos appeared when Hyperion made it's way out of the depths to possess Soúro Marathas.