Ajunta Pall is a human Jedi Master, leader of the Dark Jedi exiled from the Order after the conflict the Legends shelf calls the Hundred-Year Darkness, and the first being to hold the title Dark Lord of the Sith. He appears in Star Wars: Knights of the Old Republic (2003) as a spirit in his own tomb on Korriban.
How the Sith got their name
The exiles reach Korriban and find the Sith — a red-skinned species with its own dark side tradition. Pall manoeuvres their leader into a duel, kills him with his own sword, and is received as a god-king. The order that follows takes the name of the people it conquered. It is a neat, unpleasant origin, and it explains why "Sith" refers to both a species and an ideology.
The side quest
His actual role in the game is small and disproportionately well remembered. Pall's spirit has been in his tomb for thousands of years and has concluded that the whole enterprise was a mistake — the Sith destroyed each other exactly as an order built on betrayal must. A player who engages with him rather than fighting him can talk him into letting go, and he becomes one with the Force.
Why it is unusual
Star Wars grants very few redemptions, and almost never to the architect of an evil rather than a recruit to it. Handing that to the first Sith of all, in an optional conversation in a side tomb, is a strange and rather good decision.
Canon status
Star Wars Legends. The Hundred-Year Darkness, the Exiles and Pall have no place in current canon.