Dark Lord of the Sith (Earth-2356)

"Always two, there are; no more, no less. A master and an apprentice."

- Yoda

Dark Lord of the Sith was a title held by members of the Sith Order, known as Sith Lords. In the distant past of the Old Republic, the Sith were led by the Dark Lords during their conflict against the Jedi Order. Ultimately, the Sith collapsed in defeat as a result of infighting and constant war with the Jedi. As the sole surviving member of the ancient Sith Order, Darth Bane assumed the mantle of Dark Lord and established a philosophy known as the Rule of Two, declaring that only two Dark Lords—a Sith Master and a Sith apprentice—could exist at any given time.

The Dark Lords that succeeded Darth Bane would adhere to his teachings, maintaining the Sith Order's ranks to two members throughout the nine millennium-long reign of the Galactic Republic. Darth Sidious, publicly known as Sheev Palpatine from Naboo, was the reigning Dark Lord of the Sith during the last years of the Republic and the rise of the Galactic Empire. Darth Vader also assumed the mantle after becoming Sidious' apprentice, and was the last Sith Lord in the thousand-year lineage of Darth Bane.

List of notable Dark Lords of the Sith

 * Darth Bane
 * Darth Bane's apprentice
 * Darth Plagueis
 * Darth Sidious
 * Darth Pain
 * Darth Vader

Source

 * https://starwars.fandom.com/wiki/Dark_Lord_of_the_Sith.