World 9 (New Super Mario Bros. Wii)
From the Super Mario Wiki
World 9 is the final world from New Super Mario Bros. Wii. It is a special world unlocked by defeating Bowser in World 8-Bowser's Castle. Each of the eight levels in this world is unlocked by collecting all of the Star Coins in the world of the corresponding level number. For example, collecting all Star Coins in World 1 will unlock World 9-1. Each level features the background and a music track from one of the previous eight worlds.
The levels in World 9 are among the most difficult in the game, and none of them have midway points in them, which adds to the world's challenge (like the Special Zone from Super Mario World). The rainbow theme is similar to the Rainbow Road courses in the Mario Kart series. World 9 has a total of 24 Star Coins, which are needed if the player wants to get all five stars on their profile on the file select screen. Unlike in previous worlds, the levels can be played in any order, and no Fortresses or Castles (or levels based on them) appear in this world.
Peach alludes to this world in the first ending, where she asks Mario if she ever told him about the secret world while departing back for the Mushroom Kingdom.