Note: - this plot will only work with a reasonably lawful group, with some sense of loyalty and honour.
Lord Niavon is the supreme ruler of the country of Torridon, having taken power in a miitary coup 11 years ago. Since then he has shown himself to be a just though harsh ruler, and has ruled successfully, including making several succesful foreign campaigns against neighbouring countries. He is not disliked, though he is respected rather than loved.
Enter the PCs (who can be of any level). They will be hired by Niavon to perform a task of some sort; assuming they complete it succesfully they will be well rewarded, and offered another. Give them a little time there to build up a tradition of working for Torridon, to build up some contacts in the city and time to feel that this is becoming a familiar area.
Next, Niavon launches a major invasion against Torridon’s traditional enemy to the north, the kingdom of Siluria (this can all happen “off screen”, though the PCs may be iinvolved in some aspect of it). His army is routed, his first major defeat since coming to power. After he returns, everyone can see a change in him: he has become obsessed with conquering Siluria, is raising taxes steeply and devoting the entire resources of the kingdom to it (which is naturally unpopular). Furthermore, his behaviour is beginning to border on the psychotic - he has slain a couple of servants who have displeased him, and is increasingly irrational. This can be brought home to the PCs by sending them on a mission, preferably a difficult one. When they complete it, Niavon refuses to pay, claiming they didn’t do it properly, and driving them from the throne room by force.
At this point, the PCs are faced with a dilemma. Should they stay and hope things get better, or should they leave, abandoning all their contacts and a town and region that they’ve got to know well. Furthermore, where would they go: if they go to Siluria or another neighbouring country, they potentially have much knowledge of Torridon to offer, but they may also be assumed to be spies.
-Niavon’s psychosis could be natural; on the other hand maybe it has been induced by someone. Potential people could be the Silurians, one of his advisers who wants to gain power, or a random third party who wants to exarcerbate the conflict between Torridon and Siluria.
-Whatever the case, one of his advisers may hire the PCs to investigate the cause. They may or not find it, depending on whether or not it is natural. Perhaps two adivisers hire them, each claiming the other is a prime suspect.
-The Silurians can’t be too happy about the madness, given that it has resulted in Niavon devoting his army to attacking them, even if they caused it. Of course, if they did, they couldn’t admit it. Maybe they approach the PCs asking them to investigate and see if there is a cure. Is it treason to work for your arch-enemies when the task they want you to do is to cure your lord’s madness?
-Of course, the longer the PCs stick around investigating things, the more they’re going to have to put up with Niavon’s madness. And, of course, it may be totally natural in which case all their efforts will be to of no avail.
Note - I will put up a link to Torridon as a setting once I have written it.