Notes from my Cthulhu one-shot game:
1) After downloading the floorplan of a 10,000 square foot mansion off of
http://www.dreamhomesource.com
I came up with the following story: A wealthy philanthropist has died recently, and all of the PC's are named in the will. Each of the PC's, as well as about 5 other NPC's, has been the beneficiary of charitable causes sponsored by the philanthropist. I made up various reasons why the NPC's were there, came up with dossier's on them, and I also came up with a few stock reasons of the PC's charity causes, in case the players needed a creative jump.
PC's were created at 2nd level. I made the PC's create their own because they wanted to get a feel for the system, ground up. You may want to pre-gen some PC's, if your players are not as willing.
At the reading of the will, the PC's get to meet and interact with each other. I gave about 10 minutes devoted to role-play of some of the first meetings, etc.
The will was read. I looked up several will and testaments online to get a template (I think I used Elvis Presley's will.) I read through it, giving the characters various heirlooms, trinkets, solid cash, etc. Including some stuff that could be construed as mythos related. At the VERY end, I included four words, which I deemed the lawyer would read like the dutiful puppy he was:
XXXX XXXX XXX XXXXX. (Can't remember them - they were arabic words I looked up online)
BAM! The house was then locked up in a dimensional spell which tore the house and all its contents outside of time. In fine print, the last of the will after the words was, "I hope to see you all shortly."
PLOT: The Wealthy guy decided one day that he had spent his life helping others, and all he had to show for it was advanced age, a lot of ungrateful charities, and no hope of living. He had an advanced disease that no hospital could cure, no matter the money. So he began delving into occult means of preserving life. He read of a God worshipped by various cults who promised elternal life. (I made up an appropriate-sounding cthulhoid god, named the Trickster in translation, and made him arabic. Lovecraft did this sort of thing all the time - after all, to him, the named gods were not the only ones running around in the universe).
Wealthy guy learns the secrets of the Cult after travelling to the wilds of Jordan and Saudi Arabia. He brings home the necessary ingredients, undertakes and completes the ritual. All that is needed are the completing words, and 7 sacrifices within the grounds of the ritual. He dies, and his lawyer unwittingly completes the ritual.
THen the fun begins.
The house comes alive in various means, and begins to kill off NPC's and PC's that are left alone, or in one's or two's. I tried to get creative with totally outlandish things:
- A bathtub has running water to overflowing. Anyone who entes the bathroom to shut the water off gets pulled in. The water's surface tension is increased to the intensity of bulletproof, yet transparent rubber. Anyone trying to help the victim watches them drown.
- Rec-Room equipment like dumbbells, nordic trac's, weight-lifting seats, etc. come chasing after the pc's.
- In the kitchen, knives and forks form deadly missiles.
- In the garage, cars attempt to bust out of the garage, leaving the purple vortex outside the house trying to suck out PC's into the void
- Everyone's gotta go to the bathroom sometime. The bathroom door locks, then the bathroom gets smaller and smaller, until the person inside is crushed to a high-velocity stream of paste out of the keyhole on the other side.
- The lights go out. Anyone who wishes to go try and fix it get electrocuted at the switch box, AT THE EXACT moment the lights come back on.
- Any NPC left totally alone... just vanishes with a scream. No explanation.
This is not the complete list, but it is the highlights.
CLUES: The PC's will find small clues throughout the house - a letter on a computer hard drive, a matchbook of a foreign hotel, a book on middle eastern cults, etc. They should eventually piece together most of it - but leave the "why" a mystery.
FINALE: There is a hidden stairway behind a certain bookcase in the study (DC 25 to find.) Only by a concentrated search do they find it. In this hidden room, there is a magic circle, and several bookcases and workstations with occult books.
If the characters search, they easily find the ritual, and its unbinding chants. However, by this point, Wealthy guy has gathered enough soul-power to manifest, and do two things: (1) call two hounds of tindalos to finish the PC's off, and (2) give a villain soliloquy to stall them. He will laugh at their misfortune, thank them for helping him for a change, etc. If asked why, he tells the PC's in detail his disgust at always giving and never getting what he really wanted in return - until now. He cannot directly hurt them, but he tries to keep them from running the unbinding chants.
The hounds will arrive either the round of or the round after the chants begin. They chants take a number of rounds equal to one PLUS the number of remaining victims.

I should mention that scattered about the rec-room were some items of use - powder of ibn-ghazi, a small elder sign, etc. - things that might forestall the inevitable for a round or two.
If the PC's can have someone complete the chants and tie up the hounds long enough (this will likely mean MANY PC's losing their lives), then victory can be had, because the ritual will not be complete while someone is trying to unbind it.
Feel free to use this in any fashion, steal ideas, salt to taste, and enjoy!
In my game, only 4 pc's made it - and the one PC who was a bus driver took them all home at the end on his bus.
