The Void was a black hole-like creation in Super Paper Mario. It appeared between universes, being able to suck them up. In
The Void, about to destroy Sammer Kingdom
places where the void is large, small earthquakes happen frequently. Small earthquakes also happen in Flipside whenever the void grows larger. It was created by Count Bleck with the power of the Chaos Heart. It disappeared when Mario defeated Super Dimentio. Merlon says that Castle Bleck is within the void, or a least a place a lot like it.
History
The Void was a result of Bleck's powers and the Chaos Heart, which was a result of Peach and Bowser's marrige. While the Void naturally consumes dimensions as it encounters them Bleck holds the power to start (but not stop) the Void, seen in his battle where he utilizes smaller versions of the Void.