It's all started by frisk reseting over and over again...doing genocide and killing monsters just for fun,until one day there was an error in their timeline,due to all those resets,and as a result everything changed,when frisk fall into the underground she didn't find toriel or flowey as usual,instead of that they (frisk) found a lot of dust everywhere in the ruins,when they (frisk) checked their LV they found this in their HP "ERROR/ERROR" and their LV was 0/0,they decided to continue their way...then they found a familiar face,it was Sans...but he also changed,Sans was already bleeding,and his skull was cracked,this must be due to determination power,since he's the only one who can remember all those genocide routes,he was also confused,Papyrus has changed as well,his body was missing,only his face and scarf was flying around Sans...Sans already knew this was Frisk's fault but he didn't fight them there in Snowdin,he decided to check on other monsters,but he didn't find any...monsters was already dead...he went to Undyne's house,but too bad,she was already melting, because she couldn't control in that determination power...he tried to save her but he failed. Then he went to alphys's lab,but she was already dead...he was so confused until he met someone....that guy was supposed to be missing ages ago, but because of that error he came back from void,yep he's Gaster,he explained everything to Sans,he told him that there's no way to stop this madness,their AU is dying,maybe he's gonna be the only survivor...Gaster also showed him that machine, it's a teleportation machine,it can teleport him to another AUs,but Sans refused to leave his AU just like that,he wanted first to kill Frisk for once and all,and yep,he managed to kill Frisk by a little help from Gaster and Papyrus and they (frisk) couldn't come back since they can't reset anymore... after that Sans and Papyrus (papyrus is already dead so he's a ghost now) left their AU,and Sans promised to protect other AUs no matter what