Malevolence was a group of dark side practitioners who formed a Sith cult. They made their home on Prakith, within the crumbling fortress that once belonged to the ancient Sith Lord Darth Andeddu. Even after Andeddu's demise, the cult managed to endure for millennia.
These cultists were originally brought together by Darth Andeddu to be his followers. Following his death, they isolated themselves within Andeddu's Keep. There, they dedicated their time to studying and wielding the dark side of the Force. To replace members lost to old age, they dispatched some of their own to find new guardians, all while awaiting their master's return from death. Because Prakith was cut off from the wider galaxy after the hyperspace lane that led to it disappeared, the cult remained largely unknown. This lasted until 137 ABY, when Darth Wyyrlok III, a leading lieutenant of the One Sith Order, arrived.
In 980 BBY, Darth Bane, the last Dark Lord of the Sith to survive the New Sith Wars, journeyed to Prakith. His goal was to find Andeddu's holocron, which he had learned from an old text held the secret of immortality. During this time, the people of Prakith seemed fearful of the cult. One local even treated Bane with open hostility when he said he was looking for them. The Dark Lord proceeded to kill many cultists and then took the holocron back to his home on Ciutric IV.
In 137 ABY, Darth Wyyrlok III arrived on the planet. At least eight members of the Malevolence were tricked by Lord Wyyrlok's illusions, which caused them to see each other as enemies. Only their leader, Gerlun, was spared by Wyyrlok, but only to have his life force drained by Darth Andeddu's holocron.
The Malevolence cultists tried to stop the Chagrian Sith Lord from entering, but when Wyyrlok demonstrated his superior command of Sith abilities, many of the cultists present were defeated and killed. Gerlun, the only cultist to survive (having killed his own brother, Demtri, in the chaos), acknowledged Wyyrlok's power and pledged his loyalty to him. He then guided the Sith Lord deeper into their territory. However, Gerlun saw an opportunity to betray Wyyrlok and stole Lord Andeddu's Sith holocron from him. Unbeknownst to Gerlun, Andeddu's holocron was the key to resurrecting the ancient Sith's body. It drained his life force, using it to reanimate Andeddu's corpse. With Gerlun's death, the Malevolence cult ceased to exist.
Their training in the dark side of the Force involved using telekinesis and building lightsabers.