The Force power known as Invoke spirits was a dark-side ability, a form of Sith magic that permitted the conjuration of a darksider's spirit after their death. Typically, this was done to empower the Sith magician performing the ritual with the deceased's abilities.
Those who utilized this technique would completely dedicate themselves to the dark side of the Force, proceeding to summon a specific spirit. The spellcaster would then request that they be granted the spirit's power, resulting in an intense hatred that affected those nearby. The strength of these rituals could be increased if the user was near the physical remains of the spirit whose power was being called upon.
During the last battle of the Beast Wars on Onderon, Queen Amanoa of Iziz employed this power against the armies led by the Beast-Lord Modon Kira. By calling upon the ancient spirit of the deceased Sith Lord Freedon Nadd along with the power of her living husband, Amanoa unleashed waves of complete darkness upon both the Jedi and the Beast Riders of Onderon. Those affected were instantly burdened by the dark side, experiencing feelings of utter despair and confusion. Ulic Qel-Droma and his fellow Jedi were overwhelmed by a powerful sense of fear and oppression, which almost severed their connection to the Force. The influence of Nadd was only dispelled by the arrival of Jedi Master Arca Jeth. Through Jeth's battle meditation, the resolve of the Jedi and the Beast Riders was restored, enabling them to defeat the dark-side warriors of Iziz. Later, when Master Jeth, accompanied by Jedi and Beast Riders, raided the secret Tomb of Freedon Nadd, they discovered Amanoa there, drawing deeply from the source of her power. However, in Jeth's presence, the darkness could no longer hold; Amanoa's power diminished as the light of the Jedi drove out Freedon Nadd's influence. The Queen of Onderon then died in her daughter's arms, joining her wicked ancestors.