When sans goes left in the ruins, papyrus to capture the human, but sans remember the genocides, and then gaster appears, he tell sans remembered to help alphys to fix papyrus, and fused into a amalgamate, then he back to Snowdin again, and he fainted, papyrus to help alphys to fix sans, but he made out alive, he teleported but something went wrong, his body melted, turns to half amalgamate, he grabs a orange bandana, and he fights it.

@zer0_lover