KingD19
Shai-Gen's Enigmatic Wong
We do know he's the Sith'ari, it was stated in the Star Wars info book. Here's the info I found.
Sometime during the early history of the First Sith Empire, the coming of the Sith'ari was foretold. Inspired by the legend of Adas, the Sith'ari was prophesied a perfect being, free of all restrictions—the ultimate Sith. The Sith'ari would rise to power to lead the Sith and, according to the legend, destroy them—yet, through their destruction, make them stronger than ever.
Finally, the prophecy was fulfilled by Darth Bane, Lord of the Sith. He fulfilled the prophecy first by providing Kaan with the Thought bomb, which caused the anhiliation of the Brotherhood of Darkness, and then by instating the Rule of Two, leading to a powerful line of Sith but reducing their numbers to only two at a time.
Ever since its first mention, fans have speculated who the Sith'ari was. Some of the most notable candidates among this speculation were Darth Revan, Darth Bane, Darth Sidious and Darth Vader. The Complete Star Wars Encyclopedia, however, makes it very clear that Bane is actually the looked-for Sith'ari, thus confirming the assumptions of many fans.