World of Nothing
From the Super Mario Wiki
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The World of Nothing is the setting of Chapter 6-1 in Super Paper Mario after The Void destroys Sammer's Kingdom. The world is almost completely blank - a white screen bar the few remnants of Sammer's Kingdom which occasionally appear, sitting on a thin, black line which can be walked on. After the game is beaten, Sammer's Kingdom is restored to normal and the World of Nothing can no longer be accessed.
After the Void expands to the point of consuming Sammer's Kingdom in Chapter 6-2, Tippi warps the heroes back to Flipside. However, they notice that the door leading to the world is still there. Since they still need the world's Pure Heart, the heroes enter the door and are shocked to see that all that remains is a blank world. After walking through the world for a bit the heroes find the Pure Heart, which has turned to stone. However, Mr. L then appears and he fights them inside his upgraded Brobot, Brobot L-type. After Mr. L is defeated again, Mario and the others return to Flipside to consult Merlon about what to do with the dead Pure Heart. Shortly after they leave, Mr. L regains consciousness and is visited by Dimentio. Dimentio knocks Mr. L out, sending him to The Underwhere and reverting his consciousness to Luigi.
The destruction of an entire world was a great and terrible tragedy.