Cthulhu Mythos
Charles Dexter Ward contains the first mention of the Cthulhu Mythos entity Yog-Sothoth, who appears repeatedly as an element in an incantation. Joseph Curwen is the owner of a copy of the Necronomicon (disguised as a book labelled Qanoon-e-Islam) and there are hints of cult activities in a fishing village that refer obliquely to the events narrated in "The Festival". The story also contains references to the Dream Cycle: Dr. Willett notices the "Sign of Koth" chiselled above a doorway, and remembers his friend Randolph Carter drawing the sign and explaining its powers and meaning.
Brian Lumley expanded on the character of Baron Ferenczy, mentioned but never met in The Case of Charles Dexter Ward, in his Necroscope series, specifically Book IV: Deadspeak, where Janos Ferenczy uses the Yog-Sothoth formula to call forth whole bodies from ash remains, and to return them to that state.
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