after the 5th genocide, sans realizes what he's doing isn't helping the timeline at all, so he goes to the core to see if Gaster left anything that could help him defeat the human, but due to the lab staying the same throughout all the resets, it was old and everything stayed the same, the floor caved in and he fell into the void. He woke up in the void damaged and found gaster standing in front of him. They came to an agreement. Gaster gives him access to the timeline jumper(One of Gaster's earlier inventions) and control over the fun value. Sans wakes up in a hole near the DT Extractor and grabs the timeline jumper. He heads to the judgement hall expecting to find the human, but they were already gone, so he used the timeline jumper to jump to the next universe: Underswap. He hides in the shadows watching the human kill every monster. But that doesnt mean he sits there and lets everyone die. Any monster the human didnt get to find, Sans kills that monster for himself. Fast Forwarding the story a bit, Sans misses the human again, but in his favor, the next universe was HELP_Tale, the home of science sans. He asks Science Sans to build an artificial human made with determination, and special weapons. The human was called...Dallas. Dallas had a built-in timeline jumper and a personality of his own. Him and Sans (Now named The Jumper) grew a friendship since Sans was the first person that wasn't science sans he met. They travelled in different directions, but they sometimes ended up in the same timeline, when that happened, they would team up on the human. It never worked though, because the human played dirty, and attacks Dallas when he dodges after phase 2.