Super Mario Bros. Dimensions

Taking place days after the events of Super Mario Bros. 3, Bowser is lost around the outskirts of the Mushroom Kingdom, where he finds a mysterious flashing ring. The ring can allow one to travel to an alternate dimension, one that is much less developed and a lot gloomier than the Mushroom Kingdom. Bowser comes up with a plan to build an empire in this strange new dimension, where nobody, not even Mario, can stop him! After months of progress, Bowser's empire begins to get larger and larger, and he starts using the power of the ring to "upgrade" his minions. But during the process of mutation, a single Hyper Boo freaks out and escapes the castle and back to the Mushroom Kingdom. There it crashes into a wall, which causes amnesia onto itself. Soon after, Mario discovers the lone, green Boo. Luckily, all that it remembered was Bowser's plans. It ends up being rechristened as Polter, who has the ability to switch between the two dimensions at will, and joining Mario's side to stop Bowser's plans once again!

Résumé

Super Mario Bros. Dimensions is a fan game by LangtonLion64. It features a dimension switching mechanic that changes the level and its challenges.