The Darkness is an Outer God, and the progenitor of Shub-Niggurath.
Behind the Mythos
Lovecraft mentioned the Nameless Mist in a family tree of Cthulhu he jokingly wrote in a letter.
The Magnum Innominandum and The Nameless Mist were unified as the same entity later by other writers, notably by Lin Carter and by the Call of Cthulhu RPG. For this reason, Darkness is sometimes known as "Magnum Tenebrosum" or "The Unnamed Darkness" as a parallel to its sibling.
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.