Rilutaan: The Western Marches
Also known as the Infinite Layers of the Abyss, this plane is more accurately a collection of planes closely tied together, as each of the Abyss's "layers" is as infinite as a world unto itself. The home of demons, the Abyss is a place born of evil intent and destruction. The very plane itself is a malignant force in the multiverse. Nothing good comes from this place.
While demons have a hierarchy, at the very top are the Demon Lords. These are unique demons that wield powers to rival the gods themselves, and are rightly feared by those who know their names. Cults spring up all across the world that worship these dark masters, seeking to bring their evil plots to fruition. These powerful beings control at least one layer of the Abyss, and some of them control multiples of them. Their power is truly awesome, and not to be underestimated.
The most well known of the demon lords are:
Demogorgon, the self-styled Prince of Demons
Orcus, Demon Prince of Undeath
Yeenoghu, lord of the gnolls
Baphomet, creator of minotaurs
Graz'zt, Lord of Hedonism
Lloth, Queen of the Demonweb Pits
This is by no means an exhaustive list. There are plenty of other demon lords, and all of them are fearsome creatures in their own right.