Help with Sphere of Annihilation

[Moff Rimmer]

I want to give my players a chance to explore the dungeon where they will be able to find the sphere of annihilation before it's time, but I don't want them to be able to remove it before it's time.

Any ideas on how I can limit their abilities with the sphere of annihilation before the "end-game" without it seeming too cheesy and still allow them to find the sphere before it is necessary?

[Arcmagik]

Well, I do not know what level Tomb of Horrors is, but if you want to do that, have an NPC suggest that the talisman of the sphere is incomplete and needs something else... and they have to go in there and find it, of course its not complete yet. They clear out the Tomb of Horrors (maybe hehhehehehehe!) and later on they see the same picture from the previous tomb and find that they passed up the real sphere and they go back and get it for the final show down!

[Moff Rimmer]

I like that it "is incomplete". I am running the game in Eberron, so I may or may not be using the Tomb of Horrors. Timing could be a little tricky -- basically, I want the party to find the sphere of annihilation before they "complete" the talisman.

Thanks for the suggestion.