One big problem I see with my first post is it doesn't say much of anything useful about writing an RPG adventure. I'll try to remedy that.
Celebrim said:
1) The repeated assurance of the accounts credibility and the competance of the narrator.
The hook in most CoC adventurers in the encounter with some sort of definitive evidence, the first peice of a (possibly) much larger puzzle. The evidence should suggest some mystery which needs to be explained. The evidence is something which the larger world dismisses as a fraud or one of life's unsolvable but trivial mysteries, but which to experts like the PC's is too credible to dismiss.
2) The preparation for a journey, usually with the intention of assuring to the reader its completeness.
It should not be immediately obvious to the characters what to do with the evidence. Some initial collection of clues is required - from other experts, at a library, from newspaper clippings, at a crime scene, from the public records of a courthouse, at a museum, and so forth - before the characters even know where the mystery is leading them. Once these clues are gathered, the characters may plan thier foray, whether it be to the heart of Africa, the farthest Himalayas, or a district of town to which gentlefolk would not otherwise go.
3) The journey into the unknown and away from the modern world.
The journey is important. No matter how short it may be, whether simply down into the cellar of an abandoned house, the journey must create a separation from the safe knowable world and the unknowable darkness which they are entering.
4) Isolation of the narrator.
Once the destination is reached, the characters must rely on thier own resources. In fact, it is better if the environment is hostile to the characters presence.
5) The road which can only be travelled in a single direction. The return path - if possible at all - must be by another means, and only after some unimaginable loss.
The road here mentioned can be one of either time or distance. It's geography is often physical, a literal road, or cave, or path but just as often it is mental geography. Entrances are blocked by land slides. Bridges become washed out in freak storms. Hostile forces gather behind the characters. The conveyance the characters relied on to arrive does not return, or else only goes in one direction (like a canoe down a river, a train on tracks, or an ocean liner crossing the Atlantic). Or else, the characters realize that they have awakened something, and only by passing through some test and putting it back to rest may they safely exit.
6) The presence of unexplainable dread.
This requires probably the least explanation, but it is also the hardest 'art' to running a CoC adventure. Capturing the numinous fear of the unknown within an RPG, especially with battle-tested, world weary, rules memorizing players hardened to all mythos elements is tough. I can give little real advice on how to do it, as its often as not the little things which you scarsely thought on that freak the players out. So, to the extent that I can give advice is it is to be detailed. Make your locations small, intimate, and eerie. Avoid the grandiouse until the last possible moment.
7) The insignificance of humanity and all of its achievements. This is probably the most important one and it involves rejection of both humanism (man is important because of his singularity) and traditional religion (man is important because the universe cares about him). In Lovecraftian horror, man is not alone and not unique but the universe does not care about him, and does not even really acknowledge his presence any more than it acknowledges an insect or a mouse.
This is that moment. At some point at the end of the adventure, the characters must be confronted with the thing which is of such enormous scale that they are but gnats to it. It doesn't have to be physical scale. It can be the fact that the artifact they are holding is 300 million years old, or that the villianist cultist has been around since the time of Gilgamesh, or that humanity turns out to be not even the most common intelligent inhabitant of the earth by several orders of magnitude, or that strands of the entities DNA entered the human population at least 100,000 years ago and as such everyone on the Earth is potentially a walking monster. It's not so much the thing itself necessarily (although there is a time for great old ones and 1000' high monoliths), but the implications of the thing. Fortunately, Lovecraft is filled with examples of this. Read a few stories.
8) The dawning comprehension and the accompaning panic rather than solace which understanding brings.
Because you don't control the protagonist, this is basically the payoff of telling the story right. It helps if you have a twist, but if I knew how to write twists I'd be an author and not telling you these things.
9) The inescability of the horror. Any escape is only temporary, because once the horror has been awakened it pursues the narrator endlessly.
In an RPG, I think you can overdo this. Like all RPG's, ultimately CoC depends on the PC's triumphing so that the story can continue.
10) The eventual release and acceptance of oblivion as preferable to the torment.
And I wouldn't even worry about this. It's CoC. Any decent set of CoC rules is going to kill off characters eventually.