This is a classic plot from many different cultures (I chose one that everyone will likely be acquainted with, but not wanting to spoil the movie, I left its name absent).
In a small village there are two brothers, one good and one evil. The good one is a leader and tries to help the village and the evil one tries to harm it. Sometimes they battle. Then the evil one lures the good one’s son into the ravine and starts a stampede (with the help of some lackies.) The good one charges into the ravine to save his son, and does so but in trying to climb out, the good one asks for his brother’s help, but his brother throws him into the stampede. He dies and his son is chased out of town. Now his son is all grown up and he and some of his friends want to retake the town and be leader again (even though he doesn’t want to, he feels obligated) and they need the adventurer’s help to overthrow his evil uncle and his band of lackies.
It is a story of betrayal within the family, the severing of bonds and the fruition of tensions and hatreds that only brothers can truly understand, those formed over years of living together, being so much the same yet so much different, with driving testosterone and rampant jealousy.
The actual details of this story are unimportant in my mind—this is a psychological adventure, based around exploring emotions, forcing the characters to think and the players to roleplay. The above story can be fit into any setting, any system, and can, with a bit of work, be tailored to your own unique tastes. Explore the nature of power (beyond simply that people want it, explore why they want it, both from your perspective and from the perspective of the great minds of history).
Explore the nature of sibling rivalry—could it in fact be so intense that it could lead to something like cold-blooded murder and treason? How might the evil brother react? Is he truly evil? Does he feel remorseful? How does this betrayal affect the good brother? Is it right for him to return? Must-reads for this adventure are Shakespeare’s Hamlet and MacBeth, with particular focus on the title characters. I’m not going to lay out how you should do it or how what conclusions you should reach—this is a beginning to give you an idea, not the end. What you end up with is between you and the players.
Twists (these are optional and in my opinion would make the adventure worse).
—The nephew is in fact the evil one, seeking the PC’s as his unwitting lackies (good for PCs who are stupid and trusting and don’t want to press the limits of the good, the psyche or pretty much anything else).
—The uncle is about 20 times more complex that he seems to be. His motivations lie in hitherto unseen paths of goodness, such as a strict utilitarian approach to leadership (my brother was too kind, his mercy and compassion made him beloved but made his state weak and his laws laughable. The people were falling into decline, and needed a strong, if unpopular, leader to bring them proper morality and guidance.) (Good for PC’s who are hardline zealots in what they think the good is).
—The uncle was sunder the control of an evil wizard and cannot be held-responsible for his actions. He never wanted to kill his brother, though he is evil. Is his lesser evil justification for killing him, though he clearly as some good in him? (Good for those same zealots).
—The uncle was given dispensation and pardon to remove his brother by the king, thinking that the good brother’s mind was failing and would never abdicate his rulership (thus putting the PC’s sense of goodness at odds with their obedience, if any, to the law and rightful king).