The underworld is the subterranean plane of existence, essentially everything underground beneath the Earth. It is largely the realm of the dead, the gods that deal with them, and a vast variety of subterranean monsters.
Scholars (and many ordinary people) are fascinated by the Underworld and make extensive maps of its regions. Most can agree on a basic, general layout.
The shallowest layer of the Underworld consists of the Underdark, the feydarks, and the caverns of the Dwarves. The topography of this part of the Underworld reflects the world above; the feydarks lie under the Elflands and other fey locations, the Dwarf kingdoms under their mountains, and the Underdark everywhere else.
Of the three, the Dwarf kingdoms are the most hospitable, and many scholars and tourists interested in the Underworld visit them, though not all Dwarf kingdoms welcome so many visitors easily. The feydarks are dangerous; their denizens have too many connections to the Underdark and the land of the dead and, while not necessarily evil, are infamously capricious.
The Underdark is immense, populated by strange monsters and usually dangerous. Occasionally a relatively safe enclave emerges, although these are usually unfriendly to human visitors.
The Shadowlands, also known as Hell, Hades, the Land of the Dead, and like names, are the deep underworld realm of the dead, where Tcharnog rules and his not-always-obedient servants abide. The population of the Shadowlands includes uncounted ghosts, many sorts of revenant and undead (although when they are in the underworld, they’re just the dead, not undead), as well as the Devils, who are among Tcharnog’s servants. Tcharnog himself and most of his senior advisors are not evil (they’re lawful neutral), but Asmodeus, Tcharnog’s scheming and ambitious Chief Prosecutor, and his diabolical colleagues and minions are.
Surrounding the Shadowlands are the endless twisting caves of Pandemonium, the chaotic realm of demons and aberrant horrors. Pandemonium is a place of absolute anarchy and there is no reasoning with its wandering denizens.
In addition to being the prosecutors and bailiffs of Hell, the devils are also its armies and defenders, standing guard at its borders to repel the ravening incursions of Pandemonium. Unlike the standard D&D world, notwithstanding their diabolical evil, the typical mortal would take sides with devils against demons, if it were necessary to protect the integrity of Hell, as it is an integral component of their afterlife.
The underworld also includes subrealms of fire, in volcanic regions, populated by fire elementals and salamanders.