It's not one of his more popular stories, but I think it's the first that quotes from the Necronomicon.
Wisely did Ibn Schacabao say, that happy is the tomb where no wizard hath lain, and happy the town at night whose wizards are all ashes. For it is of old rumour that the soul of the devil-bought hastes not from his charnel clay, but fats and instructs the very worm that gnaws; till out of corruption horrid life springs, and the dull scavengers of earth wax crafty to vex it and swell monstrous to plague it. Great holes secretly are digged where earth's pores ought to suffice, and things have learnt to walk that ought to crawl.
It's actually Call of Cthulhu d20 that has it (and presumably the 5.6 version of BRP cthulhu), although other than the name "The Worm That Walks", it's implemented quite differently. But basically the same idea, becoming undead by becoming of mass of vermin in human shape.