Avernus

Avernus was the first layer of Baator and was a vast charred wasteland of rubble over which the iron towers of the Dukes of Hell stood. Legions of devils marched across its plains in continual readiness for the next battle of the Blood War. A red light suffused the sky and huge fireballs flew across the layer, randomly exploding wherever they hit.

Sayings
Sayings spoken by devils in Avernus:
 * "Avernus welcomes all."
 * "Fires of the pit!"

Government
Zariel is the current ruler of Avernus, after Lord Bel, the former lord, was demoted to being her advisor by Asmodeus. She resides in a soaring basalt citadel.

Landmarks
The Bronze Citadel is a huge fortress city, housing massive numbers of devil troops and war machines. It is constantly being added to in the form of new fortifications against attacks.

The Pillar of Skulls is a hideous landmark of trophy-skulls of those killed in the Blood War that reaches a height of more than a mile. It is very close to the entrance to the second layer, Dis.

Divine Realms

 * Draukari was the realm of the chief deity of the kobolds, Kurtulmak.
 * Tiamat's Lair located here is somehow connected to the Dragonspawn Pits of Azharul, Tiamat's main realm.

Connections to the Forgotten Realms
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 Seiveril’s Crusade.

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.