The Nightmare Chunks are dark magical purple gemstones that are actually pieces of the Dark Stone and items featured in the videogame Mario & Luigi: Dream Team.
When the bat king, Antasma, was near defeat as his plans of ruling the Dream World was about to fail, he used his remaining power to 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.