Creating an unwinnable encounter for my campaign.

This whole thing sounds extremely railroady, and, frankly, would be a big red flag for me as a player. You should never ever decide the outcome an action before the die is rolled: if you do, you are removing player agency—one of the most important, core, fundamental parts of D&D.

Pre-written narratives like this are literally the antithesis of roleplaying games, which are, by their definition, collaborative storytelling based on the outcome of dice and improvisation.

If you've already decided the outcome of a fight, even before the campaign has even started, it frankly sounds like you're more interested in writing a novel than you are interested in playing a roleplaying game. Maybe I'm being blunt, but this sounds awfully similar to the kind of things you see on /r/rpghorrorstories.

/r/dndnext Thread