Lore: Mario enters Koopa's fortress to defeat him once again, but the further in he ventures, Dangerous obstacles begin to appear. Feeling uneasy, Mario steps through the final door to encounter... Luigi? He is said to have been waiting at the door to fight with Mario. In front of the last boss's door, the two brothers pledge to fight together until they die.
Mario's Madness: https://gamebanana.com/mods/359554