I had been planning on doing a post to analyse the results of my Mythic-driven horror adventure when it was finished. MoonSylver asked about doing solo horror in the Lone Wolf Roleplaying community, and my answer there formed the basis of this post.
For my toolbox, I used the horror theme rules from Mythic Variations combined with some of the tables in the adventure generator section of the d30 Sandbox Companion (primarily Phenomena, Artifact/Relic, and Key NPC). The modified Event Focus table works brilliantly, but the chaos rules seem better on paper than in practice.
The chaos factor never falling starts to make the oracle feel a bit useless, since almost anything you ask comes back as a Yes. I hit Chaos 9 by scene 12, and the adventure lasted to scene 48! I found myself grossly inflating the unlikelihood of the questions just to make it worth using the oracle at all. This is actually a problem I have with Mythic in general; the higher the chaos factor, the more asking questions starts to feel like cheating.
The high Chaos factor also meant almost every scene was modified from the initial setup. The Altered scenes were far too frequent, but this was my fault for forgetting the change to the scene setup rule that determines which modified scene to use (1-3 Altered, 4+ under the chaos factor is Interrupt, rather than even/odd for Interrupt/Altered). The reasoning behind the change is solid, but practically every scene being modified did start to bog things down a little.
Whenever Mythic gave me a Horror event, I just rolled on the d30 Phenomena table and applied the results to the indicated N/PC in a gruesome fashion (the Random Characteristics table at the back of the Labyrinth Lord AEC would have worked well for this too; perhaps next time). Physical clues (necklace, scarab, mask, grimoire) came from the d30 Artifact/Relic table, and Key NPC introduced new NPCs as needed. These all worked perfectly, though if I'd been going for a less outlandish sort of horror, I would probably have limited the number of different Phenomena (perhaps by rolling a d6 or d8 every time a new one was indicated, and not adding new ones to the list once this die result was lower than the number that had already come up).
But that was just the easy & mechanical part. Here's how I actually made a horror adventure:
The seed of the adventure was something I'd always wanted to try, but never managed to play before. The basic premise was simple: PCs get snowed in to an old manor/castle, which is the ancestral home of at least one of them; the unseasonable weather was brought on by an evil cult who are using it to trap everyone there for their own fell purpose. The sort of horror story I was after works best with a limited number of people, so I made a few rolls on the d30 Noble Household Officers and Misc. Castle Jobs tables to populate the place and decided everyone else had fled.
I didn't worry about motivations or monsters or anything at this stage, just set the scene and let things fall out according to the oracle and random tables. The old Marquess languishing on his sickbed shut up in his tower was the only non-random element I added, though I had no idea that the cult were ultimately responsible, nor what they were even doing to him; that all came out in the course of play. He could just have easily been revealed to be a werewolf; maybe that would mean the cultists were werewolves. Or maybe it would have been the Dark Family Secret, and only a secondary horror to the main plot.
My biggest piece of advice is that, in a horror adventure, there are Things Man was not Meant to Know. And by man, I mean the PCs. Don't worry about the meaning of the clues that you find until you have enough of them that the answer seems obvious. If the clues are piling up and seeming less and less connected, don't be afraid to make some of them red herrings or limit the number of different kinds of evidence (like my suggestion for limiting types of horror results above). Some of them, though, may always remain a mystery. You can put your GM hat on later if you need to figure out for their significance yourself, but your PC may not ever be privy to this information. For example, all the 'suicides' were found with little trinkets. The PCs guessed that these somehow let the cultists will them to suicide or control their actions (especially after the necklace incident), but they never figured out that they formed a mystical link to channel evil magical energy [for Horror events] generally or that the psychic energy of the anguished deaths was feeding the Thing From Beyond at the centre of the cult -- things I decided later on out of a need for internal consistency in my game world. I could have made all the items scarabs, if I'd thought about it; the fact that two scarabs were found one after another was a mere coincidence: the kind that only happens 1 time in 30, to be sure.
My best advice for running a solo horror game would be to start with a vague concept, take it slowly, and let the tables build the mystery and horror a bit at a time. Don't try to figure out what's going on until it's too late for your PC. And don't let it tell you what the monster is until you're practically having to roll for initiative. Even the prefatory remarks in Mythic Variations say that the horror theme works best with a gamemaster, but I honestly don't think it's harder than any other kind of adventure that goes beyond hack-and-slash. I'll certainly be doing it again.