Shub Niggurath is a creature created by H.P. Lovecraft. She has never physically appeared in his works, but was first mentioned in 1928's "The Dunwich Horror".
Shub Niggurath is an antagonist that the Turtles face in Infestation 2, though referred to in-story as a male. Niggurath is brought forth by the cult leader, Bloch. Donatello discerns that this creature is actually some kind of vast, dying parasitical fungus that has festered under New York for centuries. The Turtles manage to sever the veins through which it feeds, and then drop a giant idol on it to keep it from eating them instead.