A Nightsister Force crystal was a crystal native to Dathomir that was capable of enhancing the Nightsisters' Force abilities. Nightsisters wore it as part of their elaborate headdresses. Imperial Inquisitor Xarot Korlin, who had been studying the indigenous Nightsisters' ways of using the Force, sent an agent to kill one of them to obtain one of these crystals for his studies. The agent noticed that the crystal was very cold to the touch. The agent also felt uneasy having it on their person.
It is likely that the Nightsister Charal used one during her time on Endor.
This crystal is an item in Star Wars Galaxies: An Empire Divided that is available through a quest for Imperial Inquisitor Xarot Korlin. By killing a Nightsister, collecting the crystal and then deleting the quest, players are able to retain the crystal for themselves. The crystals have no force-enhancing effect, but instead could be used as a decoration for players' houses.