The abyss
The Abyss is a massive, nearly lightless sea... home to Asharra and her many soulless children. It can be reached through the deepest, water-filled tunnels of the Underdark, or indirectly through the Dining Room, a domain of Darrmoon the great maw.

No mortal has managed to enter the abyss and return. There is no sun, and thus no light or heat. The few places there is air it is stagnant and barely breathable, and omnipresent water is full of nightmarishly huge creatures that call the realm home. A few scarce chunks of jagged rock float at it's surface, amidst a sea that seems to extend in every direction. While the children of Asharra avoid these rocky islands, they are far from safe... as the largest one is actually a massive festhall and the only place in the entire plane where there is light. It is the Dining Room, a pocket dimension belonging to Darrmoon, the great maw. His servants toil there eternally, fearing both their master and the things that lurk in the darkness outside their massive festhall almost equally.

Shoggoth by ltdbassplayer

At Asharra's decree, no god may enter the Abyss save herself and her child Ulthrogrog. All others will find that the deep tunnels and ocean rifts that would normally lead to her realm extend instead into impenetrable darkness that seems to be somehow solid. This does not prevent other beings from entering and leaving... and indeed her children often swim forth from the abyss to the material plane at her bidding. Some of the fish and sea creatures made by the sea god Enki have even managed to find their way into the abyss, but most are quickly made prey for the plane's natives. The fey and more intelligent sea creatures avoid the gateways to the Abyss entirely... and for good reason.