Dark Lord of the Sith was a title used by members of the Sith, also known as Sith Lords. Under the Rule of Two, it was applied to both the Sith Master and the Sith apprentice.
Darth Plagueis‘ apprentice, Darth Sidious, conspired to restore the Sith to power roughly a millennium after the death of Darth Bane, the Dark Lord who reformed the Sith. During the Clone Wars, Darth Maul and Savage Opress claimed the mantle of Dark Lord despite the presence of Sidious and Darth Tyranus. Sidious killed Opress and chastised Maul, reminding his former apprentice that there could only be two Sith Lords at any given time in the galaxy.
After the Galactic Republic fell, the galaxy was ruled by two Dark Lords—Galactic Emperor Darth Sidious and his apprentice, the fallen Jedi Darth Vader—during the era of the Galactic Empire. Their reign ended when Vader sacrificed himself by destroying his master, rendering the Sith extinct through the deaths of the final Dark Lords.
Despite the Sith’s prophesied destruction, their legacy endured through Sidious, who was revived through cloning and the dark side of the Force, as well as Supreme Leader Kylo Ren, Vader’s grandson. During the First Order‘s war against the Resistance, Ren aspired to rule all the galaxy as the new Emperor and Dark Lord of the Sith. Following Ren’s redemption, Sidious perished along with his followers in the Battle of Exegol as the very last Dark Lord of the Sith remaining.