The Catch: It's about 100 years into the reign of the dark lord. Where most of the scattered kingdoms are struggling to pull themselves up, he's been doing pretty well. His nation is stable, his people are happy and prosperous, and he rules from a position of strength. If you follow his laws, you're fine. If you don't, you die. His laws are firm, but usually fairly just. Those nations who try to oppose him tend to get crushed horribly, but with his combination of sorcerous and divine power, the crops of his lands prosper while those of his enemies wither, his people are healthy, while plagues tend to wash over his the lands of his enemies.
So, the idea is that the PCs are from one of the bordering nations. They're nobles, among the upper ranks, and the king has recently passed on. They're in a position to decide who is on the throne, then have to decide whether to try to oppose the dark lord, or perhaps set aside old grievances and consider joining with him. His nation's doing fairly well, his people seem genuinly happy, and he's actually not a bad ruler -- he's just not supposed to be here and usurped the throne of the rightful heirs. Other than that, he's done a better job than most rulers. Which puts the PCs into a very... tricky... situations.
See, I tend to like messing around with certain tropes. What if the 'big bad guy' actually wound up being a decent ruler? What the hell do you do when the usurper is doing a better job than anyone else would? Are you willing to tear down a happy, just society because the ruler 'isn't the rightful heir' or 'took the throne by force' or similar? What happens if your doing so makes things worse, rather than better? What if you're fully aware of this before you act?
Something I'm considering doing, perhaps...