after a few years of the hunger when the core went blew, the fire was causing the town, sans want to take revenge of undyne becuz she hit his own skull, and gets the power of his eye, so now he got a plan, he kills her guards, decapitated and ripped, sans wants to fight undyne before the another accident, sans was take her right eye and ripped off, and takes his eye, before he turns into a predatory survival.