The Dark Stone is an ancient Pi'illo artifact and it is featured in Mario & Luigi: Dream Team. The Dark Stone is a dark purple stone in the shape of a spiral with jagged edges. Unlike the Dream Stone, which was made from the happy dreams of the inhabitants of the Dream World, the Dark Stone is fueled by people's nightmares.
The Dark Stone is never seen in the game, but is mentioned by Prince Dreambert after Mario and Luigi save the 3 Pi'illos in Pi'illo Castle.
Dreambert states that there were 2 stones: one called the Dream Stone and the other called the Dark Stone. Both of these stones had the power to grant anyone's wishes as long as they possessed it.
The stones were hidden because of their great power, but Antasma managed to steal the Dark Stone. Before he was sealed into the Dream World by the Pi'illos, Antasma crushed the Dark Stone into many pieces.
These pieces became Nightmare Chunks and petrified the Pi'illo people in the Dream World by trapping their soul in these chunks.