The Otherworld are Mario Mayhem villains and the main antagonists of Gate to the Multiverse.
There are three members of The Otherworld. They are referred to as the Leader, Code 2 and Code 3. They look like the Mario sprites from (in order) Super Mario World, Super Mario Bros. 3 and Super Mario Bros. 2, but with a deep purple tint.
The Otherworld are three evil clones of Mario, who were waste products after the multiverse formed and were left stuck in its hub. Despite being stuck with very few resources, they were able to survive, and vowed revenge against the creator of the multiverse.