Shub-Niggur (The Black Goat of the Woods with a Thousand Young) is an Outer God in the Cthulhu mythos of H.P. Lovecr. P. Lovecraft. The being first appeared in Lovecraft's revision story "The Last Test" (1928); however, in Lovecraft's fiction, she is never actually described, but is frequently mentioned or called upon in incantations. Shub-Niggurath also appears in the works of other mythos authors, including August Derleth, Lin Carter, and Brian
Shub-Niggurath in the mythos
Shub-Niggurath is an Outer God in the pantheon. She is a perverse fertility deity, said to appear as an enormous cloudy mass which extrudes black tentacles, slime-dripping mouths, and short, writhing goat legs. Small creatures are continually spat forth by the monstrosity, which are either reconsumed into the miasmatic form or escape to some monstrous life elsewhere.
Of all the mythos deities, Shub-Niggurath is probably the most extensively worshipped. Her worshippers include the Hyperboreans, the Muvians, and the people of Sarnath, as well as any number of druidic and barbaric cults. She is also worshipped by the non-human species of the mythos, such as the "Fungi from [ Yuggoth]" (the Mi-Go) and the [Nug-Soth] of Yaddith.  With the proper occult paraphernalia, Shub-Niggurath can be summoned to any woodlands at the time of the new moon. However, the place from whence she comes is not known. One possibility is that she dwells at the court of Azathoth at the center of the universe. She may also live beneath the planet Yaddith, where she is served by the Dholes. It is also possible that she lives in another dimension altogether. 
Shub-Niggurath is believed to have mated with Hastur to produce the beings Ithaqua, Zhar and Lloigor, 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 possibly have mated with Yog-Sothoth to produce Nug and Yeb (though their father is more likely Hastur), and even Yig to produce Byatis (which is debatable since some believe her to be Yig's mother, though he may be the offspring of Mappo no Ryûjin).
Dark Young of Shub-Niggurath
The Dark Young of Shub-Niggurath are horrifying, pitch-black monstrosities, seemingly made of ropy tentacles. They stand as tall as a tree (perhaps between twelve and twenty feet tall) on a pair of stumpy, hoofed legs. A mass of tentacles protrudes from their trunks where a head would normally be, and puckered maws, dripping green goo, cover their flanks. The monsters roughly resemble trees in silhouette — the trunks being the short legs and the tops of the trees represented by the ropy, branching bodies. A congregation of these abominations smells like an open grave. They usually dwell in woodlands wherever Shub-Niggurath's cult is active.
The Dark Young are usually called upon to preside over cult ceremonies. One means for summoning them is found in the Book of Eibon and requires a blood offering. The ritual may only be performed in the deep of the woodlands at the darkest of the moon, and the victim must be sacrificed over a stone altar. Dark young act as proxies for Shub-Niggurath in the accepting of sacrifices and the worship of cultists, in the devouring of non-cultists, and in the spreading of their mother's faith across the world.
Gof'nn hupadgh Shub-Niggurath
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 and then regurgitates the cultist as a transformed satyr-like being. A changed worshiper is also endowed with immortal life. 
The Black Goat
Although Shub-Niggurath is often associated with the epithet The Black Goat of the Woods with a Thousand Young, it is possible that the Black Goat is a separate entity. Rodolfo Ferraresi, in his essay "The Question of Shub-Niggurath", says that Lovecraft himself separated the two in his writings, such as in "Out of the Aeons" (1935) in which a distinction is made between Shub-Niggurath and the Black Goat — the goat is the figurehead through which Shub-Niggurath is worshipped. The most persuasive distinction, however, is the depiction of the Black Goat as a male, most notably in the rite performed in "The Whisperer in Darkness" (1931) in which the Black Goat is called the "Lord of the Woods". The Black Goat may be the personification of Pan, since Lovecraft was influenced by Arthur Machen's The Great God Pan (1890), a story that probably inspired Lovecraft's "The Dunwich Horror" (1929). In this incarnation, the Black Goat may represent Satan in the form of the satyr, a half-man, half-goat. In folklore, the satyr symbolized a man with excessive sexual appetites. The Black Goat may otherwise be a male, earthly form of Shub-Niggurath — an incarnation she assumes to copulate with her worshipers.
Appearances in other fiction
Both Stephen King and Terry Pratchett have referenced Shub-Niggurath in their works. Terry Pratchett parodies Lovecraftian gods, referring to them as the things from the "Dungeon Dimensions". For example, in Pratchett's Moving Pictures, the being "Tshup Aklathep, Infernal Star Toad with A Million Young" kills its victims by showing them pictures of its children until their brains implode. Some of the Doctor Who spin-off novels have identified the Nestene Consciousness (the being which animates the Autons) as one of the offspring of Shub-Niggurath. The connection was first drawn in Millennial Rites by Craig Hinton, and has been followed up in other appearances of the Consciousness in the novels.
Additionaly light novel and anime serier "Overlord" makes a reference to her when main character cast a a spell that summon Shub-Niggurath offspring to destroy enemy army. He sacrifice 90 thousands enemy troops to summon five Dark Young and use them to slaughter an additional 90 thousands for a total of 180,000. He use this ritual in plain sight, not in the woods during new moon.
- A version of Shub-Niggurath is the final adversary of the first-person shooter computer game Quake.
- Shub-Niggurath makes an appearance as the main villain in the online PC game Arcane 2: The Stone Circle.
- In Bruno the Bandit, one of the denizens of the demon world is "Shub-Megawrath", a goat-like blob creature with a thousand children (1001, if you count the croatoan clone of Bruno she created). The character is meant to be another of the Lovecraft references that occasionally pepper the strip.
- Shub-Niggurath is the name of a French band.
- Shub-Niggurath is the name of an old Mexican black/death metal band (albums: Horror Creatures Demo, 1990; Unknown Adorer EP, 1991; Blasphemies Of Nether World EP, 1992; The Black Goatlike Arise Demo, 1993; Evilness And Darkness Prevails Full-length, 1994; The Kinglike Celebration (Final Aeon on Earth) Full-length, 1997).
- Shub-Niggurath is mentioned by the band Morbid Angel in their song "Angel of Disease".