Battle of the Gods: Arthur vs. Luigi was a cataclysmic and universe-defining event taking place over 1000 years ago. It refers to the epic clash between Arthur and Luigi to determine who was the real super duper god of punching things.
After hearing about Arthur's exploits and prowess in combat, Luigi publicly called him out and challenged him to a fight for supremacy and the title of One Punch Man by sinking the ocean city of Atlantis. Taking this as a reasonable gesture to get his attention, Arthur wrote a message to Luigi with the bodies of ten thousand warriors in the plains of North America that they should fight far away so as to avoid destroying the Earth with the sheer power exerted in their conflict. After much thought and internal debate, Luigi settled on the 63rd moon of Saturn, known as the Fist of the North Star for its peculiar shape, as the battleground, and the two warriors blasted off into space with the force of their manliness alone, reaching the moon in a few hours.
The exact details of the battle are unknown, as no one was there to witness it and both fighters barely remember what happened themselves. However, it is known that during the fight, the entire moon and several nearby asteroids were destroyed, Saturn's path through space was permanently altered, and Mars gained several giant, suspiciously fist-shaped craters.
At some point, both Arthur and Luigi decided to call it quits and squaresies since they were apparently not getting anywhere and that week's episode of Seinfeld was about to air, and the two quickly forgot about determining who was better. The matter remains unresolved to this day.