Dusttale Sans had went through so much resets that he couldn't remember, but something happened. It felt like what happened that originally sent him into the madness of Dusting. He knew what he was doing wouldn't continue to work, so he tried something... different.

Chara was attacking Sans, as usual, but Sans wasn't fighting back, but rather Dodging. An aura of ultimate power wisped around him, as he prepared for his final move.

The final move was supposed to erase the human's very being, all memories of them, all physical changes, everything. It was meant to be impossible to avoid, but when there's determination involved, you can never be certain.

As Sans attacked, Chara broke the fabric of reality in their favour, and it created a portal to another dimension: Outertale. Chara new to stay back from it, as they had been through many AUs before, but Sans, hoping to escape the endless resets ran in.

He was brought to another world not unlike his own, except it was in space. His LV craving quickly set in again, and as he was easily the strongest force there, he dusted everyone. Chara still hadn't fallen, so Sans would just have to wait for them.

He changed his clothing to be more suitable for the coldness of space Snowdin, and thrived in his 20 LV. With the desire of more destruction, he broke the lands around him, leaving only an empty planet with a snowy area.

When the human came, Sans was prepared. Now, with a LV20 Sans with bloodlust, who had destroyed an entire planet and it's inhabitants with neither sweat nor regret, against a LV1 Human, who are incapable of hitting sans with his dodging skills and the uncertainty of space.

Another thing, Frisk is freezing to death. They're slowed down greatly. Sans is less affected.

... but if he can still be affected by the cold, maybe we need to lay him rest in the snow, in a nice little Snowgrave?