I played in a campaign once where some of the gods of the Forgotten Realms were trying to manipulate Cthulhu into destroying their rivals - the setup was that the big C was far more powerful than any individual god, with an utterly alien mindset but he could possibly be manipulated via some of the primal rules of the multiverse. Turned out the manipulators didn't quite know what they were doing, and things of course got more than a little out of hand. Our PCs clashed with some Cthulhu cultists - the old one's influence on Toril grew stronger as he ate more deities - and of course got to read some musty tomes and deal with magics that were far too dangerous for any sane person to even consider using, but most of the Cthulhoid action occurred way over our heads and we only saw hints and portents of what was going on.
Basic campaign plot was that our PCs were specified ahead of time as mostly evil, and each was on the run or otherwise an outcast. Each had a unique magic item, that turned out to be much more than it seemed. Through various machinations we became aware of what was going on among the gods, and the most likely outcome (Cthulhu eats the world), so being self-serving types we looked for a way to escape. Along the way, we discovered that most of the unique magic items we carried actually held the essence of defeated gods, who were using us to hide from their rivals and also to make their escape. All except my characters item, which I ultimately discovered was a staff bound to Nyarlathotep. Which of course meant that when we did make our escape to a little-known alternate prime (a place called Earth, as it turned out), I brought along the herald of the old gods that would inevitably doom this new world, too. Fortunately, I figured it out before the rest of the group and kept it from them until we made our escape, otherwise my mighty mage probably wouldn't have survived to make the trip, but as it was he figured he had enough time to figure out some new escape route before Cthulhu and company got around to tracking us down...
Worked very well as a premise for a closed campaign (this was in University, where our group makeup changed every four months due to co-op work terms, so all our campaigns lasted that long). It hit the Lovecraft style very well, since other than one critter my mage summoned with a variant summon monster spell, all the mythos critters were hinted at rather than acting as normal D&D monsters.
Mouseferatu said:
How did you go about it? Did you tell your players ahead of time? Did you spring it on them during game-play? And if the latter, how did they react? Were there groans and complaints, or were they able to take it completely seriously?
We knew something bad was going on, but didn't figure out the Cthulhu angle until a month or so into the four-month campaign. We players thought it was pretty cool, mostly because it was handled so well - if we'd just been killing Fungi from Mi-Go for loot instead of orcs or gnolls, then it wouldn't have been quite so well-received, I'm sure. Of course, this was also 15ish years ago, before the official D&D versions of Cthulhoid deities had much prominence, might be nowadays it wouldn't have as much impact.