Mythos in Greyhawk / The Black Dog of the Cairn Hills
Been digging into my first cross-genre experiment lately - stirring a little Cthulhu Mythos into my Greyhawk. It's funny - in basing my main campaign around the Free City and the Castle dungeons, this ends up being kind of inevitable. Both Gary and Rob Kuntz sprinkled this stuff liberally throughout their modules (published and otherwise). (Rob, if you were perchance unaware, was Gary's designated co-DM and co-author of the "holy shit 24+ levels" version of the Castle dungeons, along with a metric buttload of early Greyhawk campaign material.) This also gives me a chance to play around with my Call of Cthulhu stuff that sits around gathering dust. (I have more AD&D campaigns than I have time for, never mind other systems.) Besides that, I have an evil party to whip up adversaries for, and what better than stuff that makes them look good in comparison?
And hell, we all love cults.
Currently I'm brainstorming on the my own version of RJK's Temple of the Elder Gods, the cultists of which my players have managed to run into (and piss off). Thinking of a nice mean Lovecraft-y killer that a vengeful cult leader might unleash on their adversaries (assuming force of arms, traditional magical methods, and even psionic killers had failed), I stumbled across a Gord the Rogue reference (here) to Tharizdun having created 3 yeth hounds to hunt down Gord. Which lead my mind immediately to the Hounds of Tindalos from Frank Belnap Long's story of the same name (and Mythos-approved in HPL's The Whisperer in Darkness). Now, I could go to my CoC books and convert, but The Hounds are more or less an eldritched-up version of the black dog archetype found in English folklore - one version of which (the yeth hound) Gygax saw fit to add to AD&D. Seems like a perfect fit.
The real-world yeth hound is said to be the spirit of an unbaptised infant. In sword & sorcery terms, this could be read as: Cult leader sacrifices baby (Babies? Shout out to Carcosa!), gets otherworldly ghost dog to hunt you through fucking TIME through an ANGLE (nod to the Mythos version), and you die. Sounds about right. It's especially Mythos-y in that unless you have magic, you're completely fucked if one shows up - even magic weapons only wound the hound 1hp per "plus", so only a sorceror (or a high-level priest) has any hope of killing the thing before it eats the whole party.
So don't piss off a cult leader, I guess. Or at least sharply restrict their access to live babies. Either way.