Tartarus, or Tartaros, is both a deity and a place in the underworld - even lower than Hades. In ancient orphic sources and in the mystery schools Tartaros is also the unbounded first-existing "thing" from which the Light and the cosmos is born.
The Greek poet Hesiod asserts that a bronze anvil falling from heaven would fall 9 days before it reached the Earth. The anvil would take 9 more days to fall from Earth to Tartarus. As a place so far from the sun and so deep in the earth, Tartarus is hemmed in by 3 layers of night, which surrounds a bronze wall which in turn encompasses Tartarus. It is a dank and wretched pit engulfed in murky gloom. It is one of the primordial objects, along with Chaos, Earth, and Eros, that emerged into the universe.
While, according to Greek mythology, Hades is the place of the dead, Tartarus also has a number of inhabitants. When Cronus, the ruling Titan, came to power he imprisoned the Cyclopes in Tartarus. Zeus released them to aid in his conflict with the Titan giants. The gods of Olympus eventually defeated them and they were cast into Tartarus. They were guarded by giants, each with 50 enormous heads and 100 strong arms, who were called Hecatonchires. Later, when Zeus overcame the monster Typhus, the offspring of Tartarus and Gaia, he threw it, too, into the same pit.
Tartarus is also the place where the punishment fits the crime. For example Sisyphus, who was both a thief and murderer, was condemned for eternity to push a boulder up a hill only to have it roll down at the top. Also found there was Ixion, the first human to spill the blood of a relative. He caused his father in-law to fall into a pit of burning coals to avoid paying the bride-price. The fitting punishment was to spend eternity on a flaming wheel. Tantalus, who enjoyed the confidence of the gods by conversing and dining with them, shared the food and the secrets of the gods with his friends. The fitting punishment was to be immersed up to his neck in cool water, which disappeared whenever he attempted to quench his thirst, and luscious grapes above him that leapt up when he tried to take a hold.
In Roman mythology, Tartarus is the place where sinners are sent. Virgil describes it in the Aeneid as a gigantic place, surrounded by the flaming river Phlegethon and triple walls to avoid sinners escaping from it. It is guarded by a hydra with fifty black gaping jaws, which sits at a screeching gate protected by columns of solid adamant, a substance akin to diamond - so hard that nothing will cut through it. Inside, there is a castle with wide walls, and a tall iron turret. Tisiphone, one of the Furies who represents revenge, stands guard sleepless at the top of this turret lashing a whip. There is a pit inside which is said to extend down into the earth twice as far as the distance from the lands of the living to Olympus. At the bottom of this pit lie the Titans, the twin sons of Aloeus and many other sinners. Still more sinners are contained inside Tartarus, with punishments similar to those of Greek myth.
The author of Peter's First Epistle alludes to this tradition, naming Tartaros as the judgement of fallen angels.
Rhadamanthus, Aeacus and Minos were the judges of the dead and chose who went to Tartarus. Rhadamanthus judged Asian souls; Aeacus judged European souls and Minos was the deciding vote and judge of the Greek.