Double Bad Time. Creeper Duo. (The name of their duo theme is in the title.) A Swap Papyrus was in the middle of a fight with the human when Creeper!Sans appeared through a glitch rift and fell into the fight. Seeing what was going on, Sans joined the Papyrus and fought the human. Creeper's file entering into a core spot in the game altered Papyrus's file to become faulty as well. Note: They always have a hard-mode theme going on as a hard-mode fight with Sans cannot be done normally and since Creeper's file is corrupted when he is fought, he is always in hard-mode.
Double Bad Time Hard-mode cover by KnuckleDuster on YouTube. Will create more for Creeper!Swap at a later date.