The Gaping Maw

Demogorgon's layer in the Abyss is a vast wilderness of savagery and madness known as the Gaping Maw, where even powerful demons go insane with fear. Reflecting Demogorgon’s dual nature, the Gaping Maw consists of a massive primeval continent covered in dense jungle, surrounded by a seemingly endless expanse of ocean and brine flats. The Prince of Demons rules his layer from two serpentine towers, which emerge from a turbid sea. Each tower is topped with an enormous fanged skull. The spires constitute the fortress of Abysm, where few creatures can venture without descending into madness. (Source: DMG D&D 5e)
88th layer of the Abyss.
This was a layer consisting of a great sea of briny water broken by tall, sharp, ugly, rocky promontories. Demogorgon's palace, called Abysm, consisted of two twin towers roughly shaped like tightly coiled serpents, crowned at the top with skull-shaped minarets. The two towers were linked by a bridge near the top. Most of the fortress lay underwater beneath the towers. The reefs and caverns near the fortress were home to aboleth, kraken, and ixitxachitls, which constantly warred with each other and worshiped Demogorgon in his palace above.
Demogorgon also kept a fortress in the bog of the Gaping Maw called Ungorth Reddik. This was where he housed his armies of hezrous, aboleths, scrags, skum, and ichythoid creatures. The fortress was guarded by unusual retrievers, golems, and other evil constructs. Demogorgon was also served by abyssal marauders.
The only significant landmass of the layer was a vast continent covered in tropical jungles. Here, Demogorgon's capital city of Lemoriax was located. (https://forgottenrealms.fandom.com/wiki/Demogorgon)

Type

Dimensional plane

Location under

The Abyss

Owner/Ruler

Demogorgon

Ruling/Owning Rank

Demon Lord