It starts in a geno run, Alphys made a discovery, reports showed a massive anomaly in the timespace continuum. timelines jumping left and right, stopping and starting, until suddenly, everything ends, she calls sans and to inform him about the situation, sans couldnt believe it at the start, so alphys suggested that he could get some determination and try to remember every time the timeline ends, at the end, he decided to do it, maybe he can use it to make better knock knock jokes, little did he know, it all went down.

The geno run ended, sans died like always, but then, he woke up from his bed again, a moment ago, he was bleeding on the judgement hall, and now.. hes alive, the machine was right.
Suddenly, it all comes to sans now, not only he remembers your genocide, but remembers all of them, so much information blew over his head, losing sanity every second it passes, his eye starts to melt, he lost all sense of humor, or compassion, all thats left is his death, over and over again.

With his mind on the clouds, not only he decides to take every opportunity of yours to get lv, but he wants to overflow himself with determination, maybe that way, he will be able to to something with this timeline, maybe he can return it all back to normal, but without you existing.

There is no humanity left on that skeleton.. only an empty shell of pure agony.

The more lv and determination he gets, the more big and deformed will be.