There is an important pair of fraternal twins. Possibly minor dieties, sorcerers, or maybe just an important pair of nobles. One is evil and one is good. The thing is, the one who looks evil at first glance is the good one, and vice versa.
There is a person who has a well-known reputation, probably involving something illegal. But this person is either very clever and crafty, or really like his or her privacy, for no one knows who this person actually is. As an added twist, perhaps this person is of an age that would noramlly be wildly inappropriate (say, a child is a much-feared bandit) or maybe not the expected gender or even race.
One of the most wanted assassins in the realm looks exactly like an important person (local nobility, PC, etc.) This may or may not be common knowledge.
The heroes have destroyed the minions, plunged through the catacombs and defeated the guardians. They slowly enter the chamber to find the dark mastermind behind the scheme. The mastermind has a request for them, however:
"You must destroy me. To destroy me is to rid the world of a great evil, that is to say, me. But before you can destroy me, you must understand what evil is, what evil must be, why evil must exist."