From the Super Mario Wiki
Grumblump Inferno, otherwise named World Bowser-7, is the seventh level in World Bowser from Super Mario 3D World, becoming accessible after the completion of A Beam in the Dark. It is the only appearance of the Grumblump, a block-like object that the player uses to traverse the lava. Completion of this level unlocks both Motley Bossblob's Encore and Hisstocrat Returns.
This level starts on a platform with a Grumblump nearby. The Grumblump takes the player to another one, the only difference between them being that the second Grumblump is cube shaped. After navigating past some lava pathways, the player eventually reaches another platform with two Hammer Bros. and the Checkpoint Flag. The third Grumblump takes the player to another platform which houses some Splorches. The fourth and final Grumblump eventually transports the player to the Goal Pole.
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