The Hyperborean cycle is a series of short stories by Clark Ashton Smith that take place in the fictional prehistoric setting of Hyperborea. Smith's cycle takes cues from his friends, H. P. Lovecraft and Robert E. Howard and their works. Lovecraft wrote to Smith in a letter dated 3 December 1929: "I must not delay in expressing my well-nigh delirious delight at The Tale of Satampra Zeiros [Smith's short story]... [W]hat an atmosphere! I can see & feel & smell the jungle around immemorial Commoriom, which I am sure must lie buried today in glacial ice near Olathoe, in the Land of Lomar!". Soon afterward, Lovecraft included Smith's Tsathoggua (which originally appeared in "The Tale of Satampra Zeiros") in the story "The Mound", ghostwritten for Zealia Bishop in December 1929. Lovecraft also mentioned Tsathoggua in "The Whisperer in Darkness", which he began on February 24, 1930. Because Smith in turn borrowed numerous Lovecraftian elements, the cycle itself may be regarded as a branch of the Cthulhu Mythos. In a letter to August Derleth dated 26 July 1944, Smith wrote: "In common with other weird tales writers, I have ... made a few passing references (often under slightly altered names, such as Iog-Sotot for Yog-Sothoth and Kthulhut for Cthulhu) to some of the Lovecraftian deities. My Hyperborean tales, it seems to me, with their primordial, prehuman and sometimes premundane background and figures, are the closest to the Cthulhu Mythos, but most of them are written in a vein of grotesque humor that differentiates them vastly. However, such a tale as "The Coming of the White Worm" might be regarded as a direct contribution to the Mythos.".The Hyperborean cycle mixes cosmic horror with an Iron Age setting. Adding to the peril is the rapidly approaching ice age, which threatens to wipe out all life on the Hyperborean continent. A host of other deities play important roles in the cycle; foremost is the toad-god Tsathoggua, who dwells in Mount Voormithadreth.
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