Mushroom Kingdom Fusion is a fan game with modified Mario gameplay, crossed over with many different video game universes and elements.
This is just a tiny sampling of the amount of crossover levels in this fan game. This is a rendition of the first Halo's most famous level: The Library. MKF's version is very long, and it is auto-scrolling to simulate you having to follow 343 Guilty Spark through The Library. This video also previews the unique crossover powerup: the Spartan Suit. It gives a character the guise of a Halo character, depending on the character being used.