When the Sith Order on Korriban turned on themselves after Darth Malak's death and Darth Revan's disappearance, Sion was, apparently, one of the Sith Lords that escaped to the Unknown Regions in the galaxy. But it seemed he actually went to Malachor V, where he became apprentice to Darth Traya, Revan's former teacher.
Sion was a uniquely powerful individual, having learned the ways of pain in the Trayus Academy on Malachor. Torturing and killing others enhanced his strength in the Force, and his ability to feed on his own pain and hatred made him immortal. A master with a lightsaber and perhaps the most infamous Sith assassin of the era, Sion is known to be responsible for the death of countless Jedi during the shadow war waged by himself and Lord Nihilus.
While on Korriban, he captured and killed Jedi Master Lonna Vash, was one of the members of the Jedi Council who sent the Jedi Exile into the Outer Rim to serve his sentence. Before her death at Sion's hands, she had this to say about him: "This Sith Lord, Darth Sion, is a perversion of the dark side, the likes of which I have never seen before. Although he was cunning enough to capture me, he seems like a volatile brute, and should be easily coerced." She couldn't have been more wrong, as Darth Sion cut her down easily.
He would later duel with the Exile on Korriban, and defeat him, but the Exile would escape.
Darth Sion, along with fellow Sith Lord Darth Nihilus, was a former apprentice of Traya, before he and Nihilus cast her out of the Sith Order and stripped her of her powers. He sought not only the destruction of all Jedi, but he also hated his former master, and hoped to destroy everything that she held dear. In 3,951 BBY, he was defeated by the Exile who had served Darth Revan during the Mandalorian Wars in the Trayus Core, the heart of the Trayus Academy on the ancient Sith world of Malachor V.
Having lived so long knowing nothing but hatred, he was glad to leave that evil place at last. Because of Sion's command of his own pain, lightsaber skills, and preternatural ability to survive through the dark side, the Exile was unable to defeat him hand to hand. Instead, the Exile was forced to use his powers of persuasion to turn Sion against his own beliefs, by eroding his will and convincing him that Darth Traya had no more use for him, and that by giving up the Force he would finally be free from the horrible pain and hatred which he endured. Sion gave in to the Exile's influence, sacrificing his own life to the Force in the heart of the Trayus Academy.
Shortly before his death, Sion showed he was not entirely consumed by hatred, and that perhaps he sought something more than death and destruction. He said bitterly to the Exile, "Like you, Revan touched power and turned away. The chance to heal this fractured galaxy.. and he turned away!"
Like most other Sith Lords whose names begin with Darth, Sion's name suggests another word: scion, meaning heir. In this case, heir to his master, Traya, and the legacy of the Sith. Another word possibility is fusion, due to the nature of his condition.