After the battle against X-chara in Undertale, where he stole half of Classic Sans' soul, X-chara fled again to the void that was previously X-tale, still bound by INK's paint chains. Once there, Chara started talking to Cross, trying to trick him, and succeeding:

Cross: "I don't think this is a good idea Chara. INK wanted to help us, and that Sans, he refused to fight...." X-chara: "I made you a promise Sans... I promised you that I would help you rebuild everything we lost. That's what we want, right?" Cross: ".........." X-chara: "It's what we want, it's what "they" would have wanted"

At that moment, INK (you) came running, trying to stop him, but it was too late..., the "Event-X" had been born. Now Chara and Cross were fighting each other for control of that corrupt body:

Cross: "NO! Stop!" X-Chara: "I AM FREE!"