Darth Trefitz

Darth Plagueis had an apprentice before Darth Sidious. This apprentice was Darth Xera, who was killed by Plagueis when Sidious was ready to take his place. However, Xera clung to live and was able to survive, although his health suffered for the rest of his life.

Darth Xera took a young apprentice of his own and re-named the boy Darth Trefitz. They practiced the ways of the Dark Side in secret, and partially in keeping with the Rule of Two, Trefitz killed Xera.

He then went in search of a suitable student. He briefly considered Melany Baclaw, the niece of a late Jedi Knight, but she was not strong enough in the Force. He eventually chose Melany's younger sister, Prille Baclaw. Prille was in a group of young Imperial cadets who were attacked by a mob on Coruscant. Trefitz rescued her and an injured companion; the companion died, but Trefitz was able to heal Prille's less serious wounds instantly. She immediately asked to learn how to do what he did, and she soon became Darth Inferna.

However, the Rule of Two was there for a reason, and that reason proved to be Trefitz' undoing. When Empress Shira, the true heiress to Plagueis' Sith line, set herself up as the ruler of the Stellar Imperium, she used Trefitz as a diversion. She brought Inferna into her service and when the Imperium's Sith activity was exposed, Shira blamed Trefitz. He was then killed by Darth Inferna and the Sith were able to hide for a few more years.