Callista

Callista found an 8 year old boy called Marceloi some 152 years before SW-Fans period, training the boy to be a great Sith Lord... who then some 120 years later decided that killing women and children sucked so thence decided to become one of the good guys after a chance meeting with a warrior princess named Eileen Cross and a bunch of warfare against the Callista bred bad guys known as the Vog'on. However Callista had used his DNA to create four clones, one of which was decanted and set loose on the galaxy as the evil Sith Lord Venom.

She tried to kill Marceloi and Eileen some twelve years later, casting the girl into an abyss and leaving Marceloi full of survivor's guilt. He destroyed her, but it would be many, many years before the evil perpetrated by Callista faded from the galaxy.