Said to be both the creator and the heart of the Shadowfell, Morgariel the Chained Witch (Also known as Morghos the Chained Beast) is the witch-god of darkness. She is said to be the first 'dark' goddess, outside the traditional classifications of order, chaos, or balance.
During the churning chaos of the Realmshatter when realms merged and mixed, an 'eladrin' witch discovered the pure darkness. She sought to corrupt her brethern to the darkness as well, and many minor gods and elves alike followed her dark witchcraft. With the power of the pure dark, she became its herald, now called Morgariel.
She focused the darkness into an artifact, a scepter, with which she started to cast her ultimate spell. Around the cauldron of darkness, she and her followers started to corrupt the Feywild itself and ascend Morghos into godhood. The armies of the Seelie as well as some Unseelie intervened, aided by many gods with the key ones being the elven creators, Deltus and Serenox. With the two major gods commanding the battle, the witch's forces lost, and the spell was interrupted.
With her powers, the lady Serenox cut off the half-corrupted half of the Feywild, and essentially created the Shadowfell. Lord Deltus bound Morghos in great divine chains, sealing the witch away within the heart of this new Plane of Shadow. Additionally, the archfey elders broke her scepter into three halves, for they could not destroy its darkness. Half of Morghos' followers joined the Unseelie, while the other half remained in the Shadowfell, becoming Shadow-Elves.
Morghos still has her followers, often collectively called the Dark Hunt. Even besides the forces of the Dark Hunt, there are many witches, hags, fey, and necromancers alike across the cosmos who worship Morgariel, wearing a simple chain around their necks as her symbol.
The dark worshipers and the unseelie within Môrism hold a favorable view of the Shadowfell and recognise Morgariel as a legitimate dark goddess, despite Serenox's opposition to her and the Night Mother being the one who sealed Morgariel in the first place. While not all Môrists are dark worshipers, it isn't looked down upon by the high priestesses.