"The White Ship" by H.P. Lovecraft

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.