The Underworld, also known as Hades and Hell, is a supernatural underground land.
Composed of a network of huge caverns lit by blue and red fire, the Underworld is one of the possible destinations of the souls of the dead. Under Zeus and the other Olympian Gods' rule, it was governed by a reluctant Hades, who already used a number of demonic imps, such as Pain and Panic, to help him run things. After marrying her, he also made Persephone the Queen of Hades.
Eventually, as the Olympian gods withdrew from the world, Hades finally found himsel freed of the obligation to crew the realm of the dead, running off to the lengthy vacation on the French Riviera of which he'd always dreamed. [NON-DISNEY] By the 11th century or so, the Devil was in charge of the Underworld, as documented by Dante Alighieri's The Divine Comedy, which showed definitively that the Devil's Hell was the same location as the Underworld, still inhabited by Charon and Cerberus, and still including the River Styx.
As an afterlife
Whereas in ancient times, the Underworld had been the main destination of deceased souls from the Greco-Roman world, and sometimes beyond, with many of the souls becoming semi-dormant as they sank into the River Styx, Hell was populated only by souls whom the Devil had specifically laid a claim to, either because they were evil enough to be rejected by other afterlives, or because they had entered deals with him or another demon in his service, though in the 1940's, supposedly part of the time of the Devil's ownership of the Underworld, a number of benevolent dead still resided in its cities, albeit at the cost of constantly avoiding the malevolent Yōkai now haunting the premises. By the time of Hades's return, the Underworld was a much smaller place, but still inhabited by a number of amnesiac "lost souls".
Behind the scenes
The Underworld, or Hell, first appeared in 1929 in the cartoon Hell's Bells.