Hades

The Grey Waste of Hades is an outer plane in the negative energy zone, balanced between lawfulness and chaos. A grey sky and grey, colorless earth and plants dominate this dreary realm. It has two layers, appropriately called Glooms: Oinos and Niflheim (also called Pluton). Oinos is a realm of disease and decay, spotted with withering plants and striking iron palaces. In contrast, Niflheim is rocky, with some vegetation such as pine trees, and is the home of the dead. Niflheim houses the souls of those who were not claimed by the celestials of the upper planes or the fiends of the lower planes. These souls were converted to grey, joyless larvae that leached any remaining light or joy in the realm - and were used by night hags in various rituals and as a type of currency.
For more information on this plane, please consult the 5th Edition Dungeon Master's Guide, page 63, or see the Forgotten Realms wiki: here. Note: for this lore and world, I condensed the bottom two of three layers into one.

Tor-Oin

On the first level of this plane, Oinos, which is home to decay and disease, lives the Olcrid Khemozh, the Hex Maiden. From her dusty, decrepit tower, she directs her minions of decay and disease across the planes. Surrounding this tower is a warren of hundreds of small huts that house the night hags and their dark rituals. To get to Tor-Oin, one must pass through a labyrinth of hag huts and any curses or hexes their denizens have placed for protection.

Niflheim

The second layer of the plane is the home of the dead souls, the aforementioned larvae. It is watched over by the Nisme Helī and her various pets and guardians. Helī occupies the Dark Palace in Niflheim, but can be found frequently wandering around, checking on the dead souls and keeping relative peace between the night hags that scour her land for larvae and other magical ingredients. It is a very poorly kept secret that Helī despises having to share her plane with the likes of Khemozh, and she plots with various night hags and other fiendish denizens of the plane to oust her neighbor once and for all.

Type

Dimensional plane