Sith Master (Earth-2042)

"It is your duty to learn what I know. It is my duty to learn what there is to teach."

- Darth Sidious, to Darth Vader

Sith Master was the title of a Sith individual who instructed at least one apprentice in the ways of the dark side of the Force. Traditionally, the apprentice was required to kill his or her mentor before they could train their own apprentice and advance to the rank of Master as per the Rule of Two.

Throughout Sith history, the Master held the titles Sith Lord and Dark Lord of the Sith. In Darth Revan's Sith Order, Sith Masters held the rank of Sith Lord, but only the strongest Master ruled as Dark Lord of the Sith. Under Darth Bane's Rule of Two, the guiding principle of the new Order of the Sith Lords, the Master and his or her apprentice held both titles together.

Source

 * https://starwars.fandom.com/wiki/Sith_Master/Legends