Disbelief Sans (also known as Genocide Sans) is an AU version that was created via a rumor about Sans, where he is the final boss instead of Sans in the Genocide Route.
Sans stops sans from fighting the protganist in snowdin and tries to do it himself but dies.
What happens if he was sad that Papyrus dies encountering him on the Judgement Hall? You will have to fight Sans on a Genocide/No Mercy run, Sans to become the brother who fights you in the corridor. That’s the idea behind Disbelief Sans, who has now lost all belief and faith he had that humans could be good, and now is out to get revenge for the death of his brother and friends.
Sans left eye glows blue. He is also crying. The rest of him is the same as the regular Sans. The story can range from Sans still thinking you can change until moments before death, to him truly trying to kill you. Sans is often shown wearing hoodie, and wielding a bone about as long as he is small but he is not wearing a hoodie.
His battle starts when you keep on doing genocide route, does. The reason why he would do this is because he don't want to watch his brother and his friends die over, and over, and over. So he stop Sans, and take care of the human, but he died, so Sans has to watch his friends die...