Atlach-Nacha is a Great Old One or fictional god of the Cthulhu Mythos created by author Howard Philips Lovecraft. It is a creation of his friend Clark Ashton Smith. Atlach-Nacha resembles a gigantic spider with a quasi-human face. It first appeared in Smith's story The Seven Geases where it was the reluctant recipient of a human sacrifice given to it by the toad god Tsathoggua. It dwells in a huge cavern deep beneath Mount Voormithadreth in the now vanished kingdom of Hyperborea in the Arctic, where it spends all eternity spinning a giant spider web bridging a massive chasm between the Waking World and the Dreamlands. It is believed that the completion of this web is a sign of the dawning of the End Times as the web will create a permanent link between the Waking World and the Dreamlands, enabling the monsters inhabiting the Dreamlands to enter the Waking World. Atlach-Nacha is also believed to have come down to Earth from the planet Cykranosh (or Saturn as it is presently known as) with Tsathoggua. Because of its appearance Atlach-Nacha is often known as the Spider-God(dess) and is believed to rule all spiders. There is some disagreement concerning its gender; in Smith's original story Atlach-Nacha is referred to a male, but in subsequent stories written by other authors it is implied that it is female. Whatever its gender really is, the bloated giant purple spiders of Leng are believed to be its children and servitors.

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• The Seven Geases Clark Ashton Smith
• The Encyclopedia Cthulhuiana Daniel Harms Chaosium Publications
• Atlachnophobia James Ambuehl