August Derleth, co-founder of Arkham House and an early protege of H. P. Lovecraft is perhaps most famous for his creative misinterpretations of HPL's story cycle dealing with concepts such as Cthulhu, the Old Ones and the abhorred Necronomicon of Abdul Alhazred. Derleth believed that the so-called Cthulhu Mythos needed some "good guys" to balance out the eldritch horrors provided by HPL, Clark Ashton Smith and other authors seen in the pages of Weird Tales. The infamous (and spurious) "Black Magic Quote" was the genesis of Derleth's vision:
You will, of course, realize that all my stories, unconnected as they may be, are based on one fundamental lore or legend: that this world was inhabited at one time by another race, who in practicing black magic, lost their foothold and were expelled, yet live on outside, ever ready to take possession of this earth again.
Purportedly a quote from one of Lovecraft's letters, these words actually come from a man named Harold Farnese. Farnese was not blessed with a good memory. He managed to conflate his own discussions about "black magic" and the dichotomy of good and evil and then pass them off as Lovecraft's ideas to Derleth.
Derleth took the idea of a good/evil struggle within the Mythos and ran with it. He added new deities and new concepts -- such as using the Elder Sign to ward off dark powers in much the same way as the cross is used in Hammer horror films.
While I prefer my Lovecraft to be free of these Derlethisms from a literary standpoint, I think some of Derleth's concepts might be useful to -- and to some degree are already incorporated into -- Dungeons & Dragons.
Derleth's conflict between the horrific Great Old Ones and the benevolent Elder Gods predates mankind. Through some means, the Elder Gods overcame their chaotic evil counterparts and banished them from the Earth or trapped them in some way. Cthulhu, for instance, was brought down and imprisoned within its ancient city called R'lyeh, whereas Hastur was cast out into space to be imprisoned under the Lake of Hali on dread Carcosa.
Which brings me back to the Underworld. That onion-like, many-layered dungeon-of-dungeons beneath the crust of many a D&D world. What if certain Powers of Chaos were imprisoned down there in a similar fashion to Derleth's Old Ones? Perhaps they sleep. Perhaps they are biding their time and building up their strength to break the bindings that have held them for untold eons. Perhaps like Shub-Niggurath and Ubbo-Sathla they are giving birth to new monsters and populating the deeper strata of the Underworld.
This is rife with possibilities. And even those of you who are inclined to steer away from Lovecraftian dark gods could easily substitute something more traditionally demonic in their place.