We at KORG pride ourselves in coming up with unusual ideas for events. "Who Is the Keeper?" will probably go down as the most original, weird event we've ever run, and the funny thing was we couldn't advertise it as a KORG production head of time.
The basic idea was that the players of this Call of Cthulhu event would play the whole session without knowing, until the end, who the Keeper was. That meant that whoever was serving as the Keeper would show up at the gaming table and pretend to be a player. While everyone else was trying to survive, however, the secret Keeper would be trying to get everyone else killed.
At this point you're probably wondering how the players could do anything without a Keeper to tell them where they were, what was going on, and so forth. That's where the envelopes come in - lots and lots of them. When setting up for the event we left a box full of about 200 envelopes and a sheet of instructions that said "When everyone's here open envelope number one". That envelope told them how to pick characters (which were themselves sealed in envelopes) and where they were. As the characters moved about the ship they found themselves stranded on they opened more envelopes, choosing which further ones to open based on their choices. It was in many ways like the Choose Your Own Adventure books. Unlike the books, however, everyone knew that one of the players was actually the Keeper, so cheating was less likely. Also, the Keeper would try (in subtle ways) to get the party to make decisions that would kill some of them off.
The mechanic of opening envelopes and wondering who the Keeper was added a whole new level of paranoia to a game that already has plenty. The adventure itself took place on a ship whose crew had been killed during the night by unknown forces, leaving the party adrift in the Pacific. They knew from a shared dream that one of their number was a Cthulhu worshipper who would try to kill them once R'lyeh (the island where Cthulhu is imprisoned) rose up underneath the boat. The party spent their time trying to assemble weapons (and figure out who they could trust to carry them) and find any survivors among the crew. In the end they entered a temple on R'lyeh, trying to make their way to a mystical gateway that would teleport them somewhere else.
Running this event was a blast. Sarah Marcel and Karl Allen served as Keepers during the two sessions it was run, and in neither case did the party figure out for sure who it was that was out to get them. The Keeper reveals himself (or herself) at the end of the adventure, generally after most of the party is dead or horribly wounded. Both times the revelation came as a surprise to most party members, as they had pegged someone else as most likely to be the Keeper.
One really fun feature of "Who is the Keeper" was what we called the Afflictions. From time to time an envelope's contents would tell someone they now suffered some kind of permanent effect - for example, after being bitten by a strange spider a character would receive a card that said "POISONED". As the session went on the Afflictions would pile up - one unfortunate was the recipient of three seperate cards. At various places during the adventure players who were suffering from an Affliction would be instructed to take damage, lose Strength points, and so on. In the final part of the event several of the Afflictions proved fatal. Most dramatic of these was the IRRADIATED Affliction, which caused the victim to explode just before the Keeper was revealed. One part of the hidden Keeper's job was to make sure they didn't get any of the Afflictions, while trying to encourage others to go into the areas where they would become IRRADIATED, POISONED, POSSESSED, INFECTED, and so on. Seeing the characters slowly degenerate from their Afflictions, all the while wondering just how bad things would get, made it very hard for the hidden Keeper to stifle peals of manaical laughter.