Avernus

No planar portals connect directly to the lower layers of the Nine Hells, by Asmodeus’s orders. As such, the first layer of Avernus is the arrival point for visitors to the plane. Avernus is a rocky wasteland with rivers of blood and clouds of biting flies. Fiery comets occasionally fall from the darkened sky and leave fuming impact craters behind. Empty battlefields are littered with weapons and bones, showing where the legions of the Nine Hells met enemies on their native soil and prevailed.
The archduchess Zariel rules Avernus, supplanting her rival, Bel, who has fallen out of Asmodeus’s favour and is forced to serve as Zariel’s advisor. Tiamat, the Queen of Evil Dragons, is a prisoner on this layer, ruling her own domain but confined to the Nine Hells by Asmodeus in accordance with some ancient contract (the terms of which are known only to Tiamat and the Lords of the Nine).
Zariel’s seat of power is a soaring basalt citadel festooned with the partially incinerated corpses of guests who failed to earn the archduchess’s favour. Zariel appears as an angel whose once-beautiful skin and wings have been ruined by fire. Her eyes burn with a furious white light that can cause creatures looking upon her to burst into flame. (DMG D&D 5e)
Avernus was the first layer of the Nine Hells. The most likely beachhead for any attack by demon-kind, it was the primary battleground of the Blood War: legions of devils marched across its plains in continual readiness to repel the hordes of demon invaders that sailed the River Styx into the layer.
Notable Locations: The Bronze Citadel- A huge fortress-city dozens of square miles in extent and ringed by twelve heavily defended walls. It housed hundreds of thousands of lesser devil troops and war machines. It was constantly being added to in the form of new fortifications against attacks. The Lord of the First reigned from here.
The Great Avernus Road- A massive road leading from Bel's fortress for the purpose of transporting large armies of devils swiftly to battle.
The Pillar of Skulls- A hideous landmark of trophy-skulls of those killed in the Blood War. It reached a height of more than 1 mile (1.6 kilometres). It was very close to the entrance to the second layer, Dis.
Connections- An especially high metal spire of Dis, the plane below, skewered through the haze between layers and emerged in Avernus near the Pillar of Skulls. Its spiral stairwell let devils and petitioners cross on foot between the layers, with many falls, by chance of otherwise.
A portal to Avernus was erected in the Burial Glen of Myth Drannor by Banites loyal to the High Imperceptor of Mulmaster, but under the influence of Zhentarim agents and, by proxy, a cabal of alhoon living in the ruins. The alhoon had it erected so that the devils it spawned would prevent the local phaerimms from attacking the liches while they searched the city for magic. Although the portal was closed by the Knights of Myth Drannor, the devils it had already unleashed continued to infest the ruins until the Elven Crusade led by Seiveril Miritar.
Another portal to Avernus was erected in Dragonspear Castle by a Calishite mage after Daeros Dragonspear, the castle's builder, was tricked into sacrificing himself.
Inhabitants- The layer was inhabited primarily by abishai, lemures, nupperibos, and spinagons. Imps were also common, as well as dragons, goblins, and kobolds.
Legions of devils dressed in mail stood an eternal watch on Avernus, in readiness for a sortie in the Blood War.(https://forgottenrealms.fandom.com/wiki/Avernus)

Geography

The layer was a charred wasteland of vast ashen plains covered in rubble and the occasional line of foothills and basalt mountains heaped with broken rocks of obsidian and quartz. The ubiquitous presence of rocks and boulders, some of which seemed to resemble tormented faces and shapes of creatures, rendered the terrain extremely treacherous and difficult to cross at any pace quicker than a fast walk.
A blood-red light suffused the very air and huge fireballs flew at random through the sky, occasionally impacting and exploding wherever they hit. There was no sun or stars in the Avernus sky. Travelers needed to get to shelter like a building or cave, lest they inevitably be struck.
As with all the lower planes, the River Styx ran through Avernus, with a number of offshoots and falls. Rivulets, lakes, and streams flowed across Avernus's plains and fed the Styx. The source of this blood was not known; devils claimed it probably came from all who had died on Avernus.
The layer's territory was also in a constant state of expansion by military conquest. The Styx, which at one point flowed at the edge of the layer, was later located at its centre thanks to a relentless baatezu campaigning and conquering of gate-towns along the layer's edges. (https://forgottenrealms.fandom.com/wiki/Avernus)

History

Avernus was once ruled by Tiamat after she got locked out of Vagan, she served Asmodeus faithfully. Her job was to prevent outcast devils on the layer from becoming a threat, but she performed so poorly that Asmodeus demoted her. Knowing her failure was not deliberate by reading her thoughts, Asmodeus deigned to allow Tiamat to remain in Avernus unpunished, and even gave her a chance to regain her position if she impressed him in her new role as the guardian of the main gate to Dis. This was about when she was elevated to goddess.
Avernus was then ruled by Zariel, until she was inevitably betrayed by Bel, a pit fiend general waging the Blood War and not one of the Dark Eight. He became Lord of the First. Bel's minions whispered that he kept Zariel prisoner deep within the Bronze Citadel and drained her of her hellish power, slowly turning her into a soul shell while he enhanced his might. Lacking the support of the other Lords of the Nine, bar maybe Asmodeus, Bel could advance no further, at least for the time being. During Bel's rule, Asmodeus asked Tiamat to offer covert aid to Zariel in order to prevent Bel from becoming too powerful, in a ploy orchestrated with Bel to keep Tiamat herself in check.
Some time after the Battle for Vagan, Asmodeus offered Tiamat the rulership of Avernus once more. However, to avoid disappointing Asmodeus again and to prevent a conflict with Bel, Tiamat refused, instead offering to be Asmodeus' champion and devouring all who opposed him.
Some time later, Bel fell out of favour with Asmodeus for his inability to successfully repel a demonic invasion of Avernus. Zariel reclaimed her title as Lord of Avernus following Bel's demotion. He was forced to serve as Zariel's advisor and remained one of her chief lieutenants, waiting for her reckless tactics to lead to a mistake, so he could claim his title back.
Before Zariel's second rule, Avernus was known to be a rich and civilized realm of cities and commerce. The Blood War reduced the entire layer to a blasted and abandoned wasteland, whose only functioning structures were military citadels to muster the devilish forces. (Manly from https://forgottenrealms.fandom.com/wiki/Avernus but I've edited some of the information to make it fit to my world)

Alternative Name(s)

The First Layer

Type

Dimensional plane

Location under

The Nine Hells

Owner/Ruler

Zariel

Ruling/Owning Rank

Archdevil