Darth Sion was most likely a surviving Jedi Knight of the Jedi Civil War, same as Darth Nihilus. He and Nihilus were trained by Darth Traya in the ancient Trayus Academy on Malachor V. Darth Sion and Darth Nihilus overthrew Darth Traya and stripped her of her power. Darth Nihilus then embraced the Rule Of Two as Darth Revan did, not the Rule Of One as Traya had followed. Nihilus being the stronger one, became the master, and Sion the apprentice. Some other back ground info on Sion is the at the only reason he is alive is because of an ancient dark side ability that keeps himself from bleeding to death.He learned this ancient ability in Trayus Academy. He is one of the two Sith Lords that ever knew this rare technique. He was killed by the Jedi Exile in Trayus Academy. (As seen in KotOR II) He also has one robotic arm.