You have to know about plot coupons. This is the exact way to describe the magic rock that occupies one of the teams in Joe Abercrombie’s second book, Before they are Hanged. I am not using Abercrombie as a negative example – instead his is a brilliant play on the conventions of the plot coupon convention.
To summarize, in case you’re averse to further click-throughs, a plot coupon is the magic whatsit that our team of heroes must obtain in order to
advance the plot defeat the bad guys, by trekking across the landscape. It’s a device used all over the place in fantasy – especially in the weakest examples where it’s just an excuse to churn out more pages for the readership… or as a substitute for a stronger plotline.
[A Plot Coupon story is] the kind of story where the protagonist(s) is told to go and collect a bunch of objects. It’s a very good way into a world, because it takes you all over, looking for things. Often, early on, someone will say something like, “A thousand years ago, Estragon The Dark Clown, for reasons that will never be adequately explained in this book or its many sequels, placed his power in The Funny Hat of Doom, The Big Red Nose of Darkness, the Wig of Desmond, and the Revolving Bow Tie of Light. It has been written, that only when these four objects come together will a Saviour arise to save Clowntown. You, Beppo, you must take this map (helpfully printed in the front of the book for easy reference) and nip around the book obtaining these four things (each the object of veneration by a different culture, each guarded by very different groups of people) at great cost to yourself and to the supporting cast, and then you must bring them back here.”
I am sure I could quote some devastating examples from Edding’s Belgariad, but that’s too easy. Do take the time to read what Neil Gaiman has to say about plot coupons, though. I laughed myself silly with recognition the first time I read his post.