Harrigan:
OOC: No worries on brevity -- we're not writing a novel here, though I'll admit we're off to a better start than I was
Finally, this trinket / medallion fragment -- you do remember it. It came from a deep and dangerous part of the sewers, a dank, dark place near the city's center that gave you quite a bad feeling. It was found on a grate, a bit too large to slip through. You've never been to the -even deeper- area where that grate drained...
I'd like to step in with some advice as well, seeing as this is an exercise. I didn't want to break up the story thread though, so I'll give my two cents here.
Depending on which kind of group you have and what kind of DM you are, you could also opt to let the player tell you something about the artifact. Something like. "You do know the artifact. Where did you get your hands on it, and which troubles did you have to overcome to bring it to the merchant?"
Depending on how you answer this question, the story could take several twists that no-one could foresee. But that only really works if everyone is very improvisational and willing to come up with setting/story material on the fly. I mean. You could have stolen it from a dragonhoard, escaping only because the dragon had incredible toothaches, but he noticed you and a voice in your head told you that it was gonna find you. Or you could have 'found' it in the sewers where dark robed figures were performing all kinds of horrible sacrifices, but you took it and ran, making an enemy in some strange cult. More and more could happen. In this way, I guess, the world could be built during play, heck even a character aspect could be made during play this way.
But this is a no prep, improv intensive way of playing, not suitable for all players. I work with kids, our roleplaying usually does not even have rules. When the kids are bored in the playground I just tell them that I have a big sword and a fireproof shield for them and tell them that I, the king, order my knights to find the dragon and slay it. There's a problem though, it's invisible, so they'll have to track him down somehow. Then, for at least 30 minutes, they are gone playing again, often coming back, having changed this up, being dragons themselves. So I'm used to improv. as adults though, it can be more satisfying to have some structure so as to have a coherent story. It's a matter of style, but I'd thought I'd share it.