Tartarus

Tartarus is the lowest region of the world, as far below earth as earth is from heaven. According to the Greek poet Hesiod, a bronze anvil falling from heaven would take nine days and nights to reach earth, and an object would take the same amount of time to fall from earth into Tartarus. Tartarus is described as a dank, gloomy pit, surrounded by a wall of bronze, and beyond that a three-fold layer of night. Along with Chaos, Earth, and Eros, it is one of the first entities to exist in the universe. 

While Hades is the main realm of the dead in Greek mythology, Tartarus also contains a number of characters. In early stories, it is primarily the prison for defeated gods; the Titans were condemned to Tartarus after losing their battle against the Olympian gods, and the hecatoncheires stood over them as guards at the bronze gates. When Zeus overcomes the monster Typhus, born from Tartarus and Gaia, he hurls it too into the same abyss. 

However, in later myths Tartarus becomes a place of punishment for sinners. It resembles Hell and is the opposite of Elysium, the afterlife for the blessed. When the hero Aeneas visits the underworld, he looks into Tartarus and sees the torments inflicted on characters such as the Titans, Tityos, Otus and Ephialtes, and the Lapiths. Rhadymanthus (and, in some versions, his brother Minos) judges the dead and assigns punishment.