The Dark Stone is a key object that appears in Mario & Luigi: Dream Team. It was once used for granting nightmarish or malefic wishes.
The Dark Stone, like the Dream Stone, was kept guarded by the Pi'illos in Pi'illo Island. The bat king, Antasma, stole the Dark Stone, which provoked the Pi'illos to fight him to get it back. They imprisoned Antasma in the Dream World, but before they were finished, Antasma crushed the Dark Stone, and Nightmare Chunks were scattered throughout Pi'illo Island, turning the Pi'illos into petrified stone and trapping their spirits in the Dream World.