The Darkness is an Outer God, and the progenitor of Shub-Niggurath, in a tongue-in-cheek genealogy H. P. Lovecraft invented for the Cthulhu Mythos.
Behind the Mythos
Lovecraft placed Darkness as an offspring of Azathoth, with Nyarlathotep and the Nameless Mist as Darkness's siblings, in a family tree of Cthulhu he jokingly wrote in a letter to Clark Ashton Smith. Shub-Niggurath is placed as a child of Darkness on the tree. (As an indication of how seriously he took this tree, Lovecraft made himself a descendant of Cthulhu, with "Yogash the Ghoul", "K'baa the Serpent", and "Ghoth the Burrower (one of the Little People)" also in his ancestral line.)
The Nameless Mist was identified with the Magnum Innominandum by some later writers, notably Lin Carter and the Call of Cthulhu RPG. For this reason, Darkness is sometimes known as "Magnum Tenebrosum", or "The Great Darkness", as a parallel to its sibling. (Some readers still believe the Magnum Innominandum to be a term for Yog-Sothoth.)
In The Dream-Quest of Unknown Kadath, Randolph Carter is described as falling through endless voids of "sentient blackness". It's unknown if this is related to the entity named Darkness, though much of the language used to describe the realms outside of existence such as the Ultimate Void and the Court of Azathoth relates to blacknesses, darkness, "spiral black-vortices" and such (and the Ultimate Gods are described as "tenebrous").
Lovecraft's tongue-in-cheek family tree: (HPL: Selected Letters of H. P. Lovecraft 4.617)
Azathoth / ___________________________/__________________________________ / / / Nyarlathotep The Nameless Mist Darkness (The ancient patrician / / gens Viburnia of Haec / / RESPVLICA.ROMANA) Yog-Sothoth------------+---------Shub-Niggurath / / / ____________/___________________ L. Viburnius Marco / / / Nug Yeb / / / P. Viburnius Marco / / Legatus of Legio 11., Cthulhu* Tsathoggua* station'd at Isca Silurum, / / in Britannia Secunda / / in A. D. 103 Shaurash-ho Yabou / / / / / / / Yogash the Ghoul Nush the Eternal / / / / / / / K'baa the Serpent Gilles Grenier, / / Lord of Averoigne / ** / / Viburnia-----------+-----------Ghoth the Burrower / / (one of the Little People) Hippolyte Le Sorcier, / ancestor of Llunwy of Wales Clark Ashton Smith ancestor of Owen Gwynedd and of H.P.L. * First of their respective lines to inhabit this planet. ** This union was an hellish and nameless tragedy.
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