Shub-Niggurath

Description
Shub-Niggurath is an Outer God (or Outer Goddess) in the pantheon. She is a perverse fertility deity with many forms, One of her common forms is an enormous mass which extrudes black tentacles, slime-dripping mouths, and short, writhing goat legs. But she's also seen with a much more humanoid form, similar to a human with a goat-like head. Small creatures are continually spat forth by the monstrosity, which are either consumed by the goddess or escape to some monstrous life elsewhere. She is the mate of Hastur and of Yog-Sothoth of which was born the twins Nug and Yeb. She's also the mother of Yig, and the Grandmother of Cthulhu.

Shub-Niggurath is believed to have mated with Hastur to produce the beings Ithaqua, Zhar, and J'Zahar. Hastur may also be the father of her "Thousand Young" or "Dark Young", though there is a good chance that they were spawned by fission. She may have mated with the Great Old One Yig to produce Byatis, despite the fact she's also Yig's mother.

As a Deity
Shub-Niggurath is one of the more extensively worshiped Gods. With the proper occult paraphernalia, Shub-Niggurath can be summoned to any woodlands at the time of the new moon. Being a goddess of fertility, she grants her blessings to those who perform sacrifices for her and depending on the wish, the type of ritual and sacrifice will be different. She's known to grant her followers with dark knowledge on all sorts of hexes and incantations, she can also make people fertile again, and even impregnate those who wish to host her monstrous children. Another of her rewards include vials of her milk, which has many strange healing properties but also starts mutating those who drink it into fierce horrific creatures.

But her biggest and most seeked blessing is the Rebirth. The "gof'nn hupadgh Shub-Niggurath" is the name given to the favored, once-human worshipers of Shub-Niggurath. When the deity deems a worshiper to be most worthy, a special ceremony is held in which the Black Goat of the Woods swallows the initiate through her womb and then rebirths the cultist as a transformed satyr-like being. A changed worshiper is also endowed with immortal life.

Cults and Religions
Of all the Outer Gods deities, Shub-Niggurath is probably the most extensively worshipped among cultists. Her worshippers include the Hyperboreans, the Muvians, T'yog of K'naa, and the people of Sarnath, as well as any number of druidic and barbaric cults. She is also worshipped by the non-human species, such as the Mi-Go and the Nug-Soth. The Magna Mater, or the Great Mother, is a goddess worshipped since before Roman times, and one of the early manifestations of Shub-Niggurath. She still being worshiped by various European cults under the name Magna Mater.