Stuck with an idea [closed]

I am trying to make a scenario for my friends, but I am stuck, with implementing an idea.

Imagine that we have a magic artifact, for which both the protagonists and the antagonists are looking for. The antagonists, have a very specific plan, which plan MUST succeed, so I am able to continue the story further.

(The plan, more exactly is, that they are looking for a ritual, with which they can raise a mummy, and this ritual is hidden in a crypt. They already have the mummy in question, in a completely different location, so they will only need the ritual, which can either be written on something that they can take with them, but it just as well might be written on the walls of the crypt, from where they can note it down.)

Now, the use of this magic artifact, can be anything, it just has to be in the story, so that it gives the players something to chase. I could make it, a key, that opens the crypt in question, but in that case, what if the players achieve to lay hands on this artifact, before the antagonists? If this artifact is the only way for the antagonists to reach their goals, then they must acquire it, but if the players have no chance of acquiring or holding on to this artifact, then this artifact has no point at all imo.

As an additional info, in the crypt where this ritual is located, a star-spawn dwells, which can either be hostile to the players, to the antagonists or both. (I have not decided yet about this).

Summarized: I am looking for a scope for this artifact, which isn’t too powerful, so that it won’t prevent neither the antagonists nor the protagonists from reaching their goals, but still, it shouldn’t be meaningless either.