One Sith

The One Sith, otherwise known as the New Sith Order, is a powerful order comprised of Sith lords and are major antagonists in the New Jedi Order and Legacy eras of the Star Wars franchise. The One Sith is an order of Sith lords founded and lead by Darth Krayt who sought to expand conflict across the galaxy and have the Sith regain control over the galaxy.

Rise of Darth Krayt
Darth Tenebrous, prior to his death, had predicted the rise of what he called "the One Sith" had been predicted by him and whose predictions matched the ascent of Darth Sidious, Darth Plagueis, more than they do the coming of Krayt's One Sith, although this was a direct reference to Sidious. Darth Krayt, a former Jedi Master, soon founded the organization and became the leader of a new generation of Sith lords. Coming to a galaxy ravaged by the Clone Wars, the Galactic Civil War, and the Yuuzhan Vong War, A'Sharad Hett viewed the galaxy as weak and fractured and was in need of a single will to retain order and make it strong again.

Equipped with the knowlege acquired from the Sith holocrans found on the acient Sith world of Korriban and renaming himself Darth Krayt after  the fierce creature from his homeworld, Tatooine, he re-established the Sith Order under his leadership and thus earned the title of Dark Lord of the Sith. Krayt's new Sith order differed from Darth Bane's Order of the Sith Lords in that he decided to abolish the ancient Rule of Two and instead replace it with the Rule of One - a new system where many other Sith serve under the will of the Dark Lord.