From the Super Mario Wiki
Nightmare Chunks are stones that appear in the Dream World in the game Mario & Luigi: Dream Team. When the bat king, Antasma, was near defeat, he used his remaining power shatter the Dark Stone and imprison the Pi'illos in Nightmare Chunks. This put them in a state of eternal sleep. However, Mario and Dreamy Luigi can easily break the Nightmare Chunks in-game and free the imprisoned Pi'illo. They can be imprisoned in one or more Nightmare Chunks, depending on the Pi'illo.
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