>>96216415
I was once in a campaign where everyone thought undead either never existed or used to exist but don't anymore like dinosaurs. And the campaign revolved around them reappearing across the land. It was kinda fun, especially since the DM didn't tip his hand too early.
>DM gets pissed and walks out because we were supposed to lose the fight so he could do some event to set them up as bad guys.
I'm a railroader but I never plan things around the PCs being defeated/surrendering. I've always found players get super lucky in the hardest battles. If they get their asses kicked it's usually during some filler encounter I stuck in at the last minute to make the dungeon or whatever longer.