Atlach-Nacha is a spider-like Great Old One from the works of Clark Ashton Smith. In "The Seven Geases", the monster is shown bridging a vast chasm with its web, somewhere beneath Mount Voormithadreth in Hyperborea. What will happen when Atlach-Nacha finishes his bridge of web is not known, although it is said it will cause, or contribute, to the end of the world.
In appearance Atlach-Nacha looks like a huge hairy spider, with an oddly human face. Its expression is said to be one of both doubt and enquiry. Atlach-Nacha is worshipped in the Andaman Islands, by the Spiders of Leng and by the Tcho-Tcho of South Asia. What the worshippers gain from this worship is unknown, as Atlach-Nacha hates to leave its web, even for a moment. It may not even care whether it is worshipped or not.