Super Mario 64 was released in 1996 as a singleplayer video game. Recent mods have let two people play together, but a new romhack allows up to 24 to jump online and play through the game, kicking and punching each other all the way through.
SM64 Online is a project by Kaze Emanuar, Melonspeedruns and Marshivolt. Built atop a Super Mario 64 ROM, it lets people connect online, choose a character from the Mushroom Kingdom and play the game however they want. You can do traditional stuff like just clearing stages, you can have speedrun races or even just build a giant battle royale: