Is this part of the Cthulhu mythos? Not explicitly, no. But there's nothing preventing it from being so. Up to you to decide yourself whether or not to include it.

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The picture used is "Sona-Nyl" by GeoKorf, used under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported (CC BY-NC-ND 3.0) license (https://creativecommons.org/licenses/by-nc-nd/3.0/)

To follow along: https://www.hplovecraft.com/writings/texts/fiction/ws.aspx

This feels to be in the vein of Poe's "The Domain of Arnheim", in the sense that it is less a story, with characters and a plot, and more just a description of a setting. But not nearly as comprehensive a description as what Poe accomplishes.