Darth Bane's apprentice

"I chose to pass my knowledge on to only one. I created a legacy so resilient that now, you come before me."

- An illusion of Darth Bane to Yoda

This individual was a Sith apprentice trained by Darth Bane to carry his teachings and the Rule of Two forward after a war between the Sith and the Jedi Order.

Biography
Approximately one thousand years prior to the Invasion of Naboo, the Sith Lord Darth Bane enacted the Rule of Two. This teaching mandated that there would only be two Sith at any given time: a master to embody power, and an apprentice to crave it. As part of the Rule of Two, Bane took on an apprentice and passed his teachings onto his student. His apprentice escaped the notice of the Jedi. After the Jedi defeated Bane, his apprentice carried on the Rule of Two, which allowed the Sith to survive in secret.

Behind the scenes
Darth Bane's apprentice was first mentioned in Terry Brooks' novelization for Star Wars: Episode I The Phantom Menace, based on backstory notes by George Lucas.