Nug and Yeb are a pair of Outer Gods known as the "Twin Blasphemies," whose mythological origins are contradictory across source texts. Some accounts describe them as offspring of Shub-Niggurath, others connect them to Yog-Sothoth.
Detailed descriptions of their appearance or powers are sparse; they appear primarily in genealogical lists within forbidden texts.
Much of the standing the two names hold in the Mythos rests on a genealogical chart. Lovecraft drew it into a 1933 letter to James F. Morton. There Nug and Yeb are born of Yog-Sothoth and Shub-Niggurath, with Cthulhu descending from Nug and Tsathoggua from Yeb. The chart, though, was sketched half in jest inside a letter, and no such genealogy is set down anywhere in his fiction.
Within the stories the pair appear almost solely as names called aloud. In “The Last Test” (1928) the name bursts out of a cry of alarm, and in “The Mound” (1930), which concerns the underground land of K’n-yan, they are named beside Yig and Tulu as the objects of a secret cult. Both are pieces Lovecraft reworked from someone else’s manuscript, so the names are hard to find in the fiction published under his own.
Later writers have tried to fill the blank, but the worth of this entry lies in its not being filled. A name present in the catalogue and absent from the text; a being with a place in the genealogy and no shape to it. It is the shelf slot where the call number remains and the book has gone.