The Banshee Stone was a Chaos powerful artifact of great disruptive ability that resembles a massive shard of crystal[2a] the size of a boarding torpedo[2b]. It was once embedded in the throne room of a Slaaneshi Daemon Prince, but when this Prince foolishly thought to oppose Abaddon, the great Chaos Lord took the mighty Stone from his possession.
During the Diamor Campaign, the Banshee Stone was used by Chaos Sorcerer Xorphas to disrupt the wards of the Daemonic cage located on the planet of Amethal. Xorphas performed a long and complicated ritual, fighting with the Imperial forces on the planet and in the Diamor System to destroy the cage and open there a new Warp Rift. Though he didn't complete the ritual, the damage to Daemon cage was so heavy that even the destruction of the Banshee Stone by Blood Angels Librarian Asmasael couldn't stop the process of Daemon cage disruption.