Encounter Ideas

[Guunshtaff]

The barbarian PC in the campaign I'm DMing offered to represent someone in a duel, as he had no combat skill. The PC wound up killing the other dueler and taking the scimitar he used. The next day, a member of the dead dueler's family visited the PC to retrieve the scimitar, as it was a family heirloom and no exchange of weaponry was stipulated in the terms of the duel. The PC decided to keep it regardless, partially out of his own honor system, but mostly to disrespect their culture based on past grievances, especially as he later sold the scimitar on the black market. As DM, I want to create an intriguing and subtle way for the family to enact revenge without punishing the player for brazen role playing. In fact, I would like to see this turn into a really cool side quest, but I'm hitting some writer's block. Can I get some suggestions?

[hymer]

Wouldn't the obvious choice be to either contact the authorities or begin spreading the word on him? Since he's the burly type, attacking him wouldn't be so easy, so maybe they just want to tarnish his reputation. I've no idea what the culture is like, so it's hard to be more specific.
If you want to get grim, the unarmed women in the marketplace (say) might pelt him with produce for his cowardly, irreligious and/or ignoble act.
Or have it reflect poorly on the guy he represented, and show his difficult situation - clearly grateful, but at the same time he has to bear this heavy social burden.
Or have the weapon have some significance that wasn't obvious. If this family are real bad guys, they may have needed that scimitar in some sort of nasty ritual. Or they were lying about it being an heirloom, and they need it for a deal that's going down - and now they can't find it and their plan is imploding. In that way, the barb did the best thing he could have, albeit unknowingly.
If they're actually nice people, maybe the sword is needed in blessing unions, and a teary-eyed member of the family begs the adventurers to get it back so her daughter can be legally married before anyone finds out she's pregnant.

Well, a few ideas, hope some of it might help unblock you. So to speak.

[TheThan]

Off the top of my head; I’d say the family should send the authorities after the barbarian in order to retrieve the stolen property. When that doesn’t work they send some adventurers to recover it (taking the law into their own hands). After this they should know for certain that the barbarian doesn’t have it any more (not trusting his word, after all he’s not only responsible for killing one of their families, but is also a thief).

Hrrm... another option is simply to make that scimitar keep showing up as it changes hands. Maybe the next big bad they fight wields it, having recently taken it off a fallen enemy. Maybe they find it in laying in a dragon’s horde, part of a recent addition to his collection. Or in a museum as part of an antiques collection (you said it was an ancestral blade). Maybe an NPC shows up with it, having just purchased it off the black market. Really you could go nuts with making this thing change hands, and keep coming back to the barbarian. You’ll get bonus points for writing out a lengthy story about how this thing travels around.

[Craft (Cheese)]

Well, a few pieces of information that would make this easier.

First, what kinds of things does this barbarian character care about? He sold off a sword of great sentimental value, so a good revenge would be to destroy/steal something the barbarian cares about. If you don't want it to feel like punishment, make sure it's proportionate: Don't do stuff like go after his little sister or burn down his home village or whatever.

Second, what's been established about this family aside from having a sword, that was lost, and now they're upset about it? What kinds of resources are available to them, and what sorts of methods would they be okay with using to get back at the barbarian?

Without knowing these things, here's the best idea I've come up with so far: For some reason, identical copies of this scimitar start appearing everywhere. Every shop the PCs can find seem to have several of them, and every enemy the PCs encounter have one. The PCs will most likely want to know what the frick is going on here, so they investigate the source of these weapons... but the trail takes them to someplace they never expected.

[Gildedragon]

... Or have it reflect poorly on the guy he represented, and show his difficult situation...

Hymer makes a great point there. The barbarian may complicated his employer's situation. No reason to have only one party angry at the barbarian.

Other options could be:
Enforcers coming at the barbarian's doorstep, likely to capture, not kill.
A curse falling on him, if the family has access to mages, and will not go away until the scimitar is returned. The nature of the curse should be somewhat fitting: difficulty wielding the weapon that killed the guy he dueled with, aversion to scimitars...