Dis

Dis, the second layer of the Nine Hells, is a labyrinth of canyons wedged between sheer mountains rich with iron ore. Iron roads span and wend through the canyons, watched over by the garrisons of iron fortresses perched atop jagged pinnacles. The second layer takes its name from its current lord, Dispater. A manipulator and deceiver, the archduke is devilishly handsome, bearing only small horns, a tail, and a cloven left hoof to distinguish him from a human. His crimson throne stands in the heart of the Iron City of Dis, a hideous metropolis that is the largest in the Nine Hells. Planar travellers come here to conspire with devils and to close deals with night hags, rakshasas, incubi, succubi, and other fiends. Dispater collects a piece of every deal through special provisions that are added to contracts signed on his layer of the Nine Hells. (https://www.dndbeyond.com/sources/dmg/creating-a-multiverse#OuterPlanes)
Notable Locations: The Iron Tower- Personal fortress of Dispater, Lord of the Second. It is visible from the entire layer due to its enormous height, as it rose above the haze.
The Garden of Delights- A facility that offered the illusion of sumptuous meals and pleasures of the flesh for exorbitant fees.
Mentiri- The great prison of Dis, which held those that broke the rules of the Hells. It "reformed" its prisoners by constantly tempting them into becoming evil.
Inhabitants- The streets of Dis were always crowded with parading devil nobles and workers that constantly remodelled the city at Dispater's behest. The most numerous inhabitants were abishai, imps, lemures, nupperibos, and spinagons, but also shades and other planar creatures could be found.
The city's underground dungeons were filled with petitioners, prisoners of the Blood War, and kidnapped mortals from the Prime Material plane. Their tortured screams could be heard on the surface through vents in the city walls. (https://forgottenrealms.fandom.com/wiki/Dis)

Geography

Despite being potentially infinite, the city of Dis always felt crowded and oppressive to anyone inside. The entire city was made of red-hot iron. Every wall and cobblestone burned to the touch. The material exuded a column of smoke that constantly shrouded the layer in a dark haze.
Someone who managed to leave the city could walk away from it, leaving it behind as its valley lay hidden by a ring of mountains. However, very few accounts of the city's exterior were known.
On the other hand, approaching Dis felt like a break from reality. A traveller could only arrive at the city by following a road paved with skulls. As the walls approached, one was, without noticing a transition, suddenly inside the city, with no edge or outer wall in sight. (https://forgottenrealms.fandom.com/wiki/Dis)

Tourism

The city hosts numerous markets and bazaars whose wares attract creatures from various planes. (https://forgottenrealms.fandom.com/wiki/Dis)

Alternative Name(s)

The Second Layer

Type

Dimensional plane

Location under

The Nine Hells

Owner/Ruler

Dispater

Ruling/Owning Rank

Archdevil