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Plot Twist Tip:
Twist On Three Different Levels: Encounter, Story, Campaign
A great way to organize and plan your plot twists is to look at them on three different levels: encounter, story or adventure, and campaign.
An encounter twist is a simple surprise that has no intentional affect on the larger picture of things. Its main purpose is to make the encounter interesting and entertaining. For example, the PCs spot an enemy camp and creep forward to scout it out. However, they discover the camp was a ruse to lure them away from the main group and a trap is sprung.
A story twist turns the plot on its head and changes the course of the whole adventure. For example, the PCs' employer is secretly related to a character and has evil ambitions (Luke, I am your father), or the scientist is actually an android, or the new monster allies unintentionally infect other races with a killer virus.
A campaign twist is like a story twist, just on a grander scale. Try to limit a campaign to just 1 or 2 twists, if any, otherwise things can become difficult to manage. Examples are: the war was secretly started by the *allies* for economic reasons, the recovered treasure turns out to be just one piece of a larger artifact, the prophecy is thwarted, but then it is learned because of that another, even more potentially devastating one, is triggered.