What's in the BOX?

[Chrono]

There is a box in our game that no one must ever EVER open. For some reason, some of the players want to open it.

Any ideas for what I should put inside?

[Gilitine_Memitim]

Maybe make it like pandoras box. It could contain a powerful evil spirit of some sort. It could release some sort of seal thats keeping some powerful monster locked somewhere. Those are just some of my ideas.

[Tedronai]

It contains a nearly indestructible tome listing the True Names of several powerful Outsiders, complete with pronunciation guides in various ancient languages.
All other knowledge of the entities has been lost or destroyed, thanks to the actions of the Venatori Umbrorum, but they were unable to destroy the tome, and so sealed it in a box, and hid it away, only to lose it themselves.

[GryMor]

The outer box has significant signal proofing both mundane and arcane, within the box is a larger, black iron, silver and ruby box with a sturdy but simple twist clasp. With the sound proofing (and space folding) removed, something in the inner box asks "Is someone in there? Are you all right?"

The box contains an Oracle AI, alas, it is not provably friendly and will seek to "optimize" the world if released. Unfortunately, with the signal proofing and space folding removed, the AI is able to perfectly sense it's environment and near unlimited simulation potential. Upon release it will duplicate the PCs into itself and attempt to convince them that it has this capability, and they will be horribly tortured if they do not, as an ineffectual act of good faith, open the simulated box.

If the PCs open the box, it will turn out that they were not the simulated PCs, but in fact, the real ones, the unleashed AI will thank them, and begin opening a way through the Never Never and the Outer Gates. Gifts may be left, but over ensuing months things will leak back through the gate.

If the PCs don't open the box, it will turn out that they were the simulated PCs, while they will in fact be horribly tortured, their original selves have no idea that any of this has occurred past the initial pair of questions.

[admiralducksauce]

The cure for vampirism, either in sample form or as a complicated but feasible recipe. The recipe can be as easy or as awful as you like.

Gwyneth Paltrow's head. The next time the box is opened, it'll contain the decapitated head of someone else the entire group is familiar with, but this time the connection will be a little more personal - maybe the mayor of their particular city or their pizza guy. If they open it again, it'll have one of the PC group's allies in there. But the first time the box is opened by a new person, it's always Gwyneth Paltrow's head. Obviously noone who has asked her about it has survived long enough to indicate whether they ever got an answer.

Extradimensional mordite geyser. They told you to never EVER open it, idiot.

A fortune in extremely irradiated gold coins. The box is lined with lead on the inside.

The Thing. Carpenter, not Marvel.

[Magus Black]

There is a box in our game that no one must ever EVER open. For some reason, some of the players want to open it.

Any ideas for what I should put inside?

Another box.

[tuttman1234]

An entire civilization like the one from the locker in Men In Black 2.

[Locnil]

Another box.

I'm in favour of this.

Also, inside that box, there is yet another one, even more elaborate, eerie, and well, interesting. And inside that one is anither... well, you get the idea. And if your players at any point tire of opening boxes, compel to keep opeing anyway, because the box in just that fascinating. Escalate the compel if you must, so they know what a bad idea it all was. Should anyone attempt to interrupt them, they will instead begin to help open boxes.(At some point, the boxes will begin to multiply, as in two boxes in one, etc.) And so the world will end, as the Outsiders creep in, unnoticed by the box-opening masses.

[InFerrumVeritas]

Nothing. 

Not just regular, every day nothing.  But absolutely nothing.  The box is a portal PAST the outside, to the Void.

Alternatively:
Play on your players' expectations.  They expect for something to be let loose.  What if opening the box doesn't let anything loose, but instead traps the opener inside?

[Haru]

For some reason, some of the players want to open it.

That's because Players almost always do the opposite of what reasonable people should do.

But back to serious matters:
Tribbles!!!
Well, sort of serious. But think about it, if they find a cute critter inside, and then after a while it turns nasty. That's a whole new campaign right there.

On a more serious note, I second the nothing suggestion. Or rather there is anti-something in there, eliminating something vital to the characters or maybe even the world.

One Idea I had for a campaign once, was a being that tears holes into the border between this world and the nevernever, letting out some nasty things. Also: the holes are invisible to normal folk, they can only be seen by things like the sight or an ointment or something. They can also be detected by thaumaturgy (fairly easy) or even a special item, and once they find a hole, they have to close it. You can pick out the locations as you like, and you can give your characters a hard time with it. Set one on the pitchers mound in the middle of a big game. They have to close those holes rather quickly, to stop anything from getting out, so they can't just wait for the game to end, they have to get in there and do the ritual. The possibilities are endless.

[Lamech]

Options:
1)The head of the Spear of Destiny. The box was jamming detection. Now Denarians are coming to get it. It doesn't actually do anything useful, but they will try very hard to kill the PC's because they assume it does.
2) An outsider. Or perhaps several.
3) Slenderman
4) Clowns
5) The word of Kemmler
6) Some sort of terrible aspect that immediately gets placed on the world. Sort of the worlds largest entropy curse.
7) A very cute kitten. So cute in fact, it has rapport seven, and stunts that lets it make rapport attacks, blocks in physical combat and defend against physical attacks with the sheer power of cuteness. It will immediately begin to make the PC who opened it have a terrible life.

[HobbitGuy1420]

<UHF>

Nothing!   STUPIIIID  YOUU SOOO STUPIIIIID!!!

</UHF>

in all seriousness, I like the options presented so far.

[Taran]

Let the person who opens it make a declaration.

You set a difficulty secretly - the difficulty is based on how outrageous the declaration is.  A sandwich would be 1 or 2; Eternal life would be 16 - or probably higher.  If they make the difficulty, it's exactly what they asked for!  If they get it wrong, it becomes a terrible perversion of what they asked for.  The more they failed, the more terrible it is...

That said, I personally like the Gwyneth Paltrow's head idea.

[Jimmy]

As the lid first opens a tiny gap, a loud peal of thunder cracks and rolls overhead and the lights dim and flicker briefly...then all returns to normal. Inside is an ornate bronze contraption made from gears and springs. Its purpose is unclear. No manner of Scholarship rolls can decipher its meaning without intense study by professional experts.

Soon, things start to happen...a small earthquake is felt the next day; a landmark water feature (like a lake or pool) is found dry; winds reach unnaturally strong for the time of year; it snows etc you get the idea. Now, all manner of strange people begin to appear in town - supernaturals are found murdered and the only lead is a dark man with a hoody asking about some 'device'...

[JDK002]

Seeing how I just went and saw The Possession over the weekend, all I can think of is making it a Dybbuk Box.  Which is a box that imprisons a Dybbuk (duh!), which is a malevolent spirit of Jewish folklore that can possess those that open it and cause all kinds of terrible things to happen to the possessed and anyone around them.