World 8 (New Super Mario Bros. Wii)
From the Super Mario Wiki
World 8 (also known as Bowser’s World) is the final main world in New Super Mario Bros. Wii. This world is volcano-themed, with many of the levels featuring lava-themed enemies such as Lava Bubbles. The world has a total of ten levels and is the only World to not feature a Koopaling as a boss, to have the airship level be before the castle and to have different castle and tower bosses. It also has a total of eleven exits. The background music that plays in this world was rearranged in Puzzle & Dragons: Super Mario Bros. Edition as the music for World 7.
The Fortress here leads to a fight with Magikoopa, who previously in the game is only seen enhancing the Koopalings when the heroes reach them in castles. This world also features the third and final fight with Bowser Jr., once again on an airship. The final level features the two-part final battle with Bowser, who is not once seen in the game up until that point. The Enemy Course in this world is Lava Bubbles between 8-3 and World 8-Tower on the bridge.