this is the head to head undertale AU if you don't know what it is , it's when the player does a genocide route before the pacifist route and Sans remembers the player's kills from their past and decides to fight them anyways

so your Frisk and many genocide routes have passed Until eventually Sans decided to get some help and now you're facing Chara (from another dimension still the same though , just wanting to kill you) papyrus (the same as Chara) and of course Sans (btw when it says bad time trio that just means that they're talking)