Basically The Last Stand/TLS + Betrayer of a reference of an account @Twinkling .

Lore:

In a timeline, they explored to many hours to kills some monsters in the genocide run after fought Sans, Papyrus, Undyne, and Asgore while avoiding, and after they grow bored to reset the timeline and starting to do the pacifist and breaks the barrier to let the monsters rule in peace between the humans, Sans knew something strange behind the human was planning something and he knew it, the human betrayed the monsterkind for their labor, he was furious after begin to spectate.

Sans encounters and meets the human in the snowy forest, ready to take you down to his anger.

(I'm bored to do lore, so why not? -w-)