Darth Sion

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.