So there has long been a connection between the Lovecraft’s Cthuluh Mythos and the Yellow Mythos based on Robert W. Chambers’ The King in Yellow.   While Lovecraft seems to have been enamored of Chambers’ work, his own linkages come mostly from his story “The Whisperer in Darkness,” which makes two references to Hastur.  The first is simply a long line of random names and ideas associated with the Mythos.  There is a second reference, which occurs in one of the last letters received by Albert Wilmarth from Henry Akeley.  The relevant passage is as follows: 

Actually, they [the Mi-go] have never knowingly harmed men, but have often been cruelly wronged and spied upon by our species. There is a whole secret cult of evil men (a man of your mystical erudition will understand me when I link them with Hastur and the Yellow Sign) devoted to the purpose of tracking them down and injuring them on behalf of monstrous powers from other dimensions. It is against these aggressors—not against normal humanity—that the drastic precautions of the Outer Ones are directed. Incidentally, I learned that many of our lost letters were stolen not by the Outer Ones but by the emissaries of this malign cult.

The cult of Hastur is never really observed in the story and is largely tangential to the plot (beyond allegedly intercepting letters…which Akeley had initially and probably correctly attributed to those in thrall to the Mi-go.)  All in all, this is some thin gruel to bind the two cosmologies together, and yet it has stuck.  

The connection was re-enforced by August Derleth who re-organized the Mythos such that Hastur was presented as Cthuluh’s brother and rival.  I guess this is kind of cool, but it is also kind of derivative, which is how I see a lot of Derleth’s work.

All in all, I suspect the bond of the two fan-verses would probably have died off as one of many of Lovecraft’s odd passing references if not for two things.  First, the lead story in The King in Yellow, “The Repairer of Reputations,” is a seriously fucked up.  The whole book is great, but that story will stick with you.  It leaves so many niggling questions that a reader can’t let it go.  The second reason is less literary.  The Yellow Mythos was incorporated into the Call of Cthuluh role playing game.  In my mind, this bound the two Mythoses together more effectively than the work of either Lovecraft or Derleth.

Thus, we can still see the occasional Mythos adjacent work building on the linkages.  I’ve noted this in Charles Stross’s Laundry Files, and in the Oddjobs series by Heide Goody and Iain Grant.  Derleth’s version of Hastur makes an appearance in Brian Lumley’s Titus Crow stories.

Still, there are many more examples of authors working within the Lovecraft framework not bothering to incorporate the Yellow Mythos.  There are no such references in Lovecraft Country.  Jonathan L. Howard does not bother with the Yellow Mythos in either his Johannes Cabal stories or Carter and Lovecraft nor does Peter Clines in Terminus.

My own work is something of an inversion of contemporary Lovecraftian fiction.  I ground my work clearly and firmly in the Yellow Mythos, with Lovecraftian elements bolted on as oblique references (Note: I will never name Chtulhu in any work of fiction).  Part of this is a deep love for Chamber’s work, but it also provides a fresh way to play with a shared set of concepts.