Shub-Niggurath is an Outer God (or Outer Goddess) in a pantheon. She is a perverse fertility deity, said to appear as an "evil cloud-like entity". An enormous 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 consumed 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 Unknown.
With the proper occult paraphernalia, Shub-Niggurath can be summoned to any harmonically aligned spaces at the time of resonant celestial alignments. 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 Unknowns. It is also possible that she lives in another dimension altogether.