Luigi appear near the room beyond the 30-star door in the basement, which leads to the ninth level of the game, Dire Dire Docks from the game Super Mario 64. Luigi go to the endless hallway and notice Mario running in his direction in terror and says Do it too while a giant disembodied head of Isaac chases him, biting viscously while laughing maniacally

Isaac-Apparition:You want fun Luigi? ISAAC SHOW YOU FUN LUIGI! WAAAA! ISAAC NUMBER ONE! ISAAC'S THE WINNER! WAAAAHAHAHAHAHAHAHAHAHA!!