The most reliable source for the public demo is the official Mario Multiverse Discord server or the Mario Multiverse Archive on Itch.io .
Gaining access to the full version usually requires passing a quiz on their Discord or being "scouted" by the development team for your level-design skills. 2. Mario in the Multiverse (SM64 ROM Hack) Mario Multiverse Game Download Beta
The game is famously in a "perpetual beta" state. While the full version remains closed to a small circle of testers to avoid DMCA takedowns and maintain quality, a Public Demo was recently released. The most reliable source for the public demo
It includes a "Theme Maker" to create custom visual styles and an "Enemy Maker" for personalized boss encounters. Mario Multiverse Game Download Beta