After (insert number) genocide routes, Sans somehow remembers everything the human did. He tells Papyrus, and Papyrus tells Undyne. When the human is about to kill Toriel, Sans teleports into the ruins and summons bones to protect her, and helps her evacuate. Papyrus and Undyne both help everyone evacuate and the human only manages to kill 4 more monsters (probably just Aaron, Jerry, Woshua, and Snowdrake) before every other monster has evacuated. The human is at lv 10. Once the human gets to the judgement hall, they meet Sans, Papyrus, and Undyne. There will be invincibility frames, and no karma.

Once you drained Sans, Papyrus, and Undyne's energy, you try to kill Sans, but accidentally hit the ketchup located in his chest. It looked like was bleeding, but he took the ketchup off of his chest and drank it to restore some energy. This would act as a phase 1.5 where Sans doesn't let you have a turn, (sorta like Undertale Last Breath phase 3) and dies after he uses his last energy that he had just restored. Papyrus and Undyne are left to fight you for phase 2, and soon Papyrus dies and Undyne is left for phase 3. Undyne is able to get the other main character monsters to help her kill you and take your soul.