From the Super Mario Wiki
Nightmare Chunks are stones that appear in the Dream World in Mario & Luigi: Dream Team. When Antasma was near defeat before the events of the story, he used his remaining power to 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 by jumping into them from below or hitting them with their hammers to free the imprisoned Pi'illo. They can be imprisoned in one or more Nightmare Chunks, depending on the Pi'illo, in which case all Nightmare Chunks must be broken.
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