DM Bait & Switch examples?

[Grim Reader]

Any interesting examples of DMs introducing elements in an encounter or campaign that completly changed things? The big bad is the kidnapped princess etc?

Trying to gather some surprises for a campaign here.

[PotatoNinja]

Have literally everybody they think is a friend turn out to be working against them in some way.

Make the Big Bad, not such a bad guy, give him plenty of admirable traits and limitations. I always found the big "Rawr i'm evil for the sake of evil" to be very boring. Give him a family, a love interest, make him respect important good aligned NPCs.

Have the Big Bad actually be the good guy for some crazy reason. OH MY GOD YOU'VE BEEN WORKING FORTHE WRONG SIDE THE WHOLE TIME can really unnerve players. Just find ways to justify his actions behind the scenes and slowly reveal these justifications in a way that humanizes him, and then builds him up as a good guy fighting against an even bigger bad guy whose EVEN MORE SUBTLE AND DICKISH.

Really the best bait and switches build up the expectations of the players in one direction, while slowly building up the opposite in reality. It all boils down to deceiving the players one way, and bringing them to a sudden realization later. This sudden realization may come from several hints, but it almost always hits in asingle moment. Some of the best ways to do this is by running character who emulate real people and who have multiple and diverse complex reasons for their actions. Often higher powers are involved who can twist the arms of those involved in ways they otherwise may not act, or, there may be a higher CONCEPT of something that a NPC believes in or cares about that makes them act a certain way.

A man blows up an entire city with a nuclear like magical attack, killing 150,000, because the city sat atop a magical item that could not be destroyed otherwise. Moving that many people away would have tipped the hand of the guy doing the destruction and put the project in danger, showing somebody else where the item actually was (previously hidden). This Magic item could have been used to destroy the lives of millions.

Little things like that :p

[Biffoniacus_Furiou]

Many years ago I ran a game for an extremely powerful solo PC, he was the descendent of a powerful Hellfire Wyrm that's imprisoned under an ancient castle. There was a wizard BBEG making a ritual to free and control the wyrm, needed one of the wyrm's descendents as a sacrifice to complete it. BBEG tells the wyrm about his plans, knowing the wyrm can contact the PC seeking help, effectively setting the bait for a potential sacrifice.

PC learns of the wizard's plans, decides he's going to perform the ritual himself to get a pet hellfire wyrm. Has to do a few adventures to accomplish this, needs help, gets told to find an ally in a specific location. Finds a tower there, fights his way in, finds a few amiable NPCs. The shadow pouncing assassin half way up the tower was the intended ally, which was made crystal clear to him. He finishes exploring the tower anyway, finds a nonhostile Cheater of Mystra Lich at the top (of the old LN Mystra). He presumes this guy must be the intended ally because he's at the end of the adventure, and tells him everything.

They go on to defeat the BBEG Wizard and the Lich takes the initiative to nominate himself as the one who will decipher the ritual for the PC. In the meantime the PC doesn't want to be the sacrifice so he goes looking for another descendent of the wyrm. He finds a heroic Harper who had been killed and raised and couldn't be used as a sacrifice, so he seduces her to make a new descendent. The child is born, he takes it to the lich and asks what to do. Lich: "Give me the child." He hands it over, and the lich teleports away. The wyrm tells the PC that there's a twin, he goes back upstairs to find the mother and second child gone. He tracks them down to Silverymoon in the Silver Marches, gets warned that Elminster is in town, and rushes in anyway.

Fifteen in-game years later, his apprentice (an Ur-Theurge) is capable of casting 9th level spells and True Resurrects him. No gear, no wealth, but he's back in the castle, so he runs downstairs to find the wyrm gone. His former apprentice informs him that the lich came back, there was nobody to stop it so it did the ritual and rode away on the wyrm's back. Saddened and defeated, he tracked down his child, now fifteen years old, raised to believe that someone by the PC's name had murdered his father. He got to deliver a classic, "No... I am your father," and that was the end of the last session of that game

TL;DR Solo PC gets told to go to a tower to find an ally. He finds the intended ally half way up the tower, and finds a lich at the top of the tower. He presumes the lich is the intended ally even after being told otherwise, tells it everything that's going on. Lich plays along without really ever helping, eventually betrays him and steals the big prize at the end of a very long campaign. To add insult to injury the PC immediately chased after a Harper knowing she'd run into Elminster's arms for protection and got blown to smithereens.