The Asylum: Descent Into Madness
The Asylum is not a convention game, but rather a one-shot I ran on two separate occasions in honor of Halloween 2011.
The basic story revolves around an abandoned insane asylum 'just outside of town' (the town is never really specified). The founder, his son, and many of the staff were cultists of Nyarlathotep before it was shut down, and used the insanity of the patients as 'fuel' for their spells and ceremonies.
Some years back they attempted a powerful spell to summon Xipe Totec, the avatar of Nyarlathotep who would come to this dimension and prepare the way for Nyarlathotep's arrival. But the spell went awry, patients escaped and killed townsfolk, and the asylum was shut down, leaving the summoning spell perpetually unresolved...
...until a member of a government surveying crew stumbled inside and suffered a serious head wound. The blood from his cut energized the spell just enough that Xipe Totec could reach out and influence those with vulnerable minds, those prone to mental instability. Those being the Player Characters of course!
This motley band of broken people converges on the asylum, losing sanity as they explore, until finally Totec has enough power to break through the portal to this world and confront them. Strangely enough, this always seems to happen about a half-hour before the end of game time. The PCs either defeat the still-vulnerable form of Xipe Totec and save the world, or they are all killed, allowing him to go forth and gather enough power to bring Nyarlathotep to our dimension.
How It Went
The first run-through had 7 players, and I think suffered because of it - we all still had a good time, but everything seemed very rushed and chaotic with so many characters vying for attention.
The second run-through had only 5 players and went much smoother. I made two big changes the second time around:
First, I shortened the very beginning of the game, when each PC is experiencing individual weirdness in their lives, to get the group together faster. This was kind of a rookie mistake on the first run. Oops.
Second, I made the group's shared nightmare of the asylum much more compelling - instead of just feeling sort of "the asylum feels like home, like somewhere you want to be", the characters were confronted by dead people from their pasts, begging and pleading with them to come to the asylum and "set them free". Much more effective!