you were walking around snowdin After Meeting with papyrus.another skeleton that’s shorter then the other Blocks the way. It’s sans! Papyrus told you about Sans and how he wants to be a royal guard “mwehehe! Stay back Human! I the magnificent Sans! Will capture you and become a royal guard!” you teleport to a black void and you become black and white. Sans is also black and white. He summons some bones and starts attacking you (BTW! You can Chose the Passive ending or the genocide ending!)