i had this idea for a game. i think it would work best as a one shot or just a few games.

the idea is that  horror movie about an evil demon is going to bring in the real thing. there is a prophecy telling how it can be stopped. anyway here is my rough draft of an intro:

Urgent Notice to All AHRO members! EMERGENCY PRIORITY!

Nightmare of ledgen:

prophecy of the nightmare:

I am the terror of the night.
i am nightmare itself
i am your collective bad drea m
tis i who haunts you when you sleep.
remember the prophecy, happenings three
for the it is i am released.
the image of death shall fill the minds and light and darkness shall combine (a horror movie is released containing a spell)
the crowds will riot,but never speak, the atrocity of which i seek    (the movie will be so awful people will protest its violence, but will abstain from sayin the evil spell)
the sun will set and hearts will sink, and in lust for power skies will blink (a promotion for it will be shown on the clouds)
be forewarned i lie in wait
make your choice, seal your fate

prophecy of light:

the beast of ledgends will be sealed
when these conditions are fullfilled:
first the chosen few must rise (the protest group)
then the evil one must die      (the one who made the evil film?)
the stones of power must be found (the stones of power were known to the ancients. they are created when cosmic energy strikes an ordinary stone)
and the city with light we must surround (?)
the two true lovers must unite (?)
and water must a fire fight (?)
the ceremonial flames must burn (?)
and to the earth peace must return (?)
from the scrolls they need to learn (?)
the gathering must then occur (?)
and of their words they must be sure (?)

my name is andy. I am the leader of the Anti horror resitance organization or AHRO. recently it has come to my attention that a horror movie is in production by the name of "nightmare". in it is a spell found of a scroll, near a body found next to a church in washington. this spell contains the power to release a beast known as the nightmare upon the world. when all the fans of the movie say it to themselves, untold horrors could occur. we must act fast and stop this! My researchers have found this scroll, "the prophesy of the beast" in circulation of the internet. also in circulation is the prophesy of light. bothe of these are provided to you, the members of the anti-horror resistance organization, with my annotations. there is still more to learn, but we still have time.the fate of the world is in our hands.

questions?

where is the third scroll?
who are the true lovers?
can we stop this before it happens?
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what do you guys think? i like that the 2 in 1 invitation/sourcebook thing.

i was thisking to have a really simple spell to reseal him. like "we are here, and we refuse evil" and a plot point could be "how can something so simple kill this thing?

You could possibly build a game around the concept, but it might be just as easy to write it as a module for an existing system.

It's extremely linear if you're only providing one win / lose scenario, and I wonder about the inclusion of a plot point that occurs 'after' the conclusion.  That would only make sense if you planned on continuing the story (I suppose it could play into the horror movie trope where the monster reappears in the last 30 seconds of the film - but it feels kind of weak for that). 

Have i Lots for You......
* A slightly different setting:
The Movie is released, but instead of one great nightmare, a "small" is brought forth by each chant of the Spell. The idea is, that these will congregate to bring forth the "Great".

* Third Scroll? Whoah! This is asking questions not explained in your Text! (I think you just forgot it.)
It can be tattooed in the back of a beast, or currently only inscripted in a (forgotten) Mayan Temple's walls. However, i'm more thinking of the "Nature" of this Third Scroll, i mean:
Is it about bending the nightmare(s?) to your own will, or do it offer an alternative to killing the Movie Maker, or will it turn the current Setting into the setting i suggested, or will it seem to make it better, but really make it worse, or just the opposite?
Or will it offer a kind of "the Nightmare never happened" when done correctly?

* Is the two Lovers really ghosts that inhabit the Scrolls of nightmare and light? (Is both original scrolls written on Human Skin Leather?)
Or even wierder, the Movie Maker and the Nightmare?
Or will it involve a Romeo & Juliet-like story?

Do you want/need more?

First off, nice idea and storyline.  Go for it!
Second, I have produced a few paintball scenario games and used this very concept, concept mind you, and enjoyed writing it.
Thirdly, for most one shots I plan I try to flesh out as much as possible and always get a "twist" near the end.