Fiction:The Firmament/Darkreach

Darkreach is the largest body of water of the plane of Irkalla and the domain of the demon lord Uzgoroth.

Description
Darkreach is an immense ocean of black, polluted waters. It is so vast that the realms of other demon lords comfortably float in its sky; Sanguine Hill, Betrayer's Rise and the Bloatgrounds are among such realms. A portion of the River Styx pierces through Darkreach, and far beneath this realm lies the Abyssal End, the prison of Varagarash Narahil.

Inhabitants
The Deep Waters are inhabited by all sorts of aquatic demons and fiendish sea monsters fathered by Uzgoroth. Any attempt to create permanent buildings in the realm ends with them being either crushed by massive waves or destroyed by the native demons.