Chara wondered why their soul was colorless. It took time for Chara to realize why their soul was different. They started acting unusual, which led to them harming Asgore and everybody else. Undyne attempted to stop Chara, while everybody evacuated the battleground. Sans eventually took over after Undyne did the best she could to defeat them. Turns out Chara was being possessed and influenced and Sans realizing this meets Frisk, a version that controls six other human souls when he goes looking for Chara who is with Asriel. Turns out Frisk lured them there in a petty revenge ploy for an alternate reality wrong caused them by the Dreemurr family.
Sans (he calls himself Epic!Sans) has some changes in his behavior. Also, he isn't lazy and always keeps his promises. He is Asgore's childhood friend, and It's insinuated he got Tori and Asgore together because he's referred to as the king's wingman. He likes to say Bruh often, sometimes replacing words with it and generally acts like an anime obsessed nerd similar to Alphys. Going so far as to give himself the title of Epic!Sans. He tends to be a huge flirt to anyone he finds cute and has a cookie addiction. Whenever he's around his best friend Cross he often engages in meme related antics. He can be serious and shows an unexpected maturity when doing so even though he still uses his favorite phrase. Often times the left socket where his scar is will be empty, a purple eye light only spawning when he's emotionally high strung or intimidating. He is 45. He is the royal scientist and is the boss of Mettaton and Alphys and is currently deceased.