The renowned master thief Slith has stolen the Ruby Sceptre from the archmage Bryseis. Unfortunately, disposing of it is causing him quite a few problems. No honourable mage will touch it - everyone knows that it is stolen goods - and unfortunately those who would take it seem to want it badly enough not to bother asking for it. Rather than paying the high price that Slith would have demanded (not that he got the chance), they seem to just be planning to take it from his dead body. Not to mention the fact that Bryseis herself wants it back rather badly. Of course, he could just give it back, but a great thief like Slith does have his pride.
Where the PCs come in
The PCs have been hired by Bryseis to find Slith and get back the Sceptre. What happens to Slith is irrelevant - she doesn’t care whether he’s alive or dead, providing she gets back her Sceptre.
Tracking Slith is not going to be easy. He is a master of stealth, and is likely to be better than anyone in your party (the group should definitely have one rogue or ranger type person in it). He should probably also have devices that protect him from being scried upon. In fact, if it were not for the fact that there are around 30 different groups chasing him at the moment (see below), the odds are that he would get clean away. Even so, you will likely need to be subtle to get him; possibly but not necessarily setting a trap of some sort (e.g. pretending to be from a wizard who is willing to buy the artifact and arranging a meeting). Note that Slith is very intelligent.
Naturally, Bryseis hasn’t just left such an important job to you. She’s hired about 20 such groups, of varying capabilities, and sent them all after Slith. Many of these other groups will cause you difficulties: the poorer ones may blunder around, accidentally warning Slith that pursuit is near, whilst the better ones may catch him before you, doing you out of the sizeable reward. Some of the less scrupulous of these people won’t hesitate at sabotaging the competition (i.e. you) through misdirection, traps, or other foul means. Depending on your party, they might choose to use such means as well.
In addition to the “friendly” competition, at least half a dozen (maybe more) dark (or at least hostile) mages have sent similar groups after Slith in order to gain the Sceptre for themselves. Some of the weaker ones may have come themselves. These people will not balk for one second at slaughtering you if they get the opportunity, or think that you may be about to make off with the sceptre. Some of the dark mages will also probably have summoned demons of various sorts to pursue Slith and bring them the artefact: demons are notoriously non-discriminating about their prey. On no account must these mages take the Sceptre or else Bryseis may never regain it: it would be worth letting Slith get away (for a while) rather than letting the Sceptre fall in to their hands.
Once you’ve found him
Even once you’ve found him, getting the Sceptre from Slith will not be easy. Though not a conventional fighter, Slith can certainly take care of himself and is adept at the sort of “striking from stealth with deadly accuracy” type of combat. He is almost certain to have an abundance of daggers etc. about his person, along with a few deadly poisons.
Slith unfortunately possesses a slight amount of magic (what self-respecting thief doesn’t?). While he certainly can’t make the Sceptre blast away a wall, he will be able to make it kill a person. Of course, at first he won’t have much of a clue how to use it and his accuracy will be essentially random, but the longer he remains holding it, the better he’ll get. In any case, charging a man who’s effectively holding an instant death ray isn’t that great an idea, even when there’s a 90% chance that he’ll miss. There is also the point that Slith is unlikely to know of the Sceptre’s instability when used frequently (see Ruby Sceptre page): the last thing you want to do is to have him make it blow up.
A final difficulty is the possibility of a multi-way battle breaking out; for example, you confront Slith. Then, a group of baddies from dark mage A who have been following you (knowing you’re on Slith’s trail) burst out from behind you and join the fray, whilst simultaneously a ravenous demon sent by dark mage B comes in from elsewhere.
-It is possible that Slith could be convinced to hand back the Sceptre, providing some way to salvage his honour was concerned. However, this is unlikely, particularly as Bryseis refuses to consider giving him any money (and any money the PCs can raise is negligible compared to his asking price).
-The PCs may well fail this mission if they are unlucky or don’t think well. This is fine: it’s not a succeed or die mission (though some might die, even if they succeed), so there should be a real possibility of failure.
-Slith isn’t particularly evil or nasty. He doesn’t kill for the sake of it. My image of him is as the archetypal “heroic thief” figure: heroic because he steals so well, but you never get a sense of villainy from him (despite the fact that what he does is obviously wrong).
-If the PCs are foolish enough to try to keep the Sceptre after getting it from Slith then redo this plot replacing “Slith” with “PCs”.