10 Video Game Boss Fights That Broke The Rules
3. Sans - Undertale
To be completely fair to Undertale, the game as a whole is entirely concerned with breaking rules: you're conditioned from early on to expect the unexpected, with its meta tone continually recontextualising RPG tropes.
Players who decide to kill everything in sight and complete Undertale's ultra-challenging Genocide Route will end up facing off against Sans as the game's true final boss - and he sure is a "bad time," just as he promises players who continue their slaughtering ways.
The game's boss fights are largely defined by the rules of bullet hell games - that is, avoiding obstacles within a confined box.
But in the Sans fight this is all thrown out, with both the constantly adjusting size of the box and Sans' ability to attack you even while you're navigating the UI.
It is, in a word, completely ridiculous. To make matters even crazier, to finally defeat Sans you need to wait a few minutes for him to fall asleep and then push the combat box to the bottom-left corner of the screen over the "Fight" option.
Even for a game so thoroughly invested in inverting RPG formula, the Sans fight is like nothing else you've ever played.