This Master Villain disguises himself as some other sort of villain. Long ago, he was given an ugly curse -- he longs for death but can never die unless slain by heroes unaware of his curse. (Naturally, the way the curse works, he has to defend himself when attacked by the heroes.) So this villain works hard to make sure the best heroes in the world have sufficient cause to want to come and kill him. He'll insult them, ruin them, kidnap or murder their loved ones, whatever it takes to bring them against him. Often, he can only die -- his curse can only be undone -- in one specific holy place, so he'll have to lure the heroes to that place to face him. If the heroes are doing research on the villain all this time, they may find out his secret, leading to a sad and painful end to the episode as the unkillable villain has to leave and find someone new to kill him.