The Dream Stone is a Zeta/Omicron exclusive item. It is ultimately used to change a Pokémon's current ability into its Dream World ability.
Dream Stones are used as currency by the Dream Lord and Dream Duchess in order to make the change possible. The Dream Lord can be found in Caspian City in the Vesryn Region while the Dream Duchess is found in Centauri Town in the Aroma Region; either NPC will change a Pokémon's ability to their Dream World ability in exchange for the stone. Dream Stones can be purchased in bulk from the Blackfist Department Store for 10,000 each.
A stone that glows with mystic power.