Dark Lord Of The Sith

The title of Dark Lord of the Sith originally referred to the ruler of the Sith Empire and later was taken by the recognized leader of the Order of the Sith Lords. Before Skere Kaan, there was usually only one Dark Lord of the Sith at a time, who was the head of the Sith Order. The major exception to this trend was the Dark Council of Vitiate's Sith Empire: all twelve Dark Councillors held the title of Dark Lord at the same time. In more recent periods, it was a title shared by multiple individuals simultaneously. The term referred to individuals of both sexes, though Lumiya claimed the title of Dark Lady of the Sith.

The title Dark Lord of the Sith is often thought to be synonymous with the title Darth. Contrary to this, however, is the fact that Sith Apprentice's have been known to carry the title as well. Two examples would be Darth Malak as Darth Revan's Sith Apprentice, and Darth Vader as Darth Sidious'. Many Sith Lords have used the Darth title, but there's no evidence as to the synonymic relationship of these two appellations.

 Dark Lord Of The Sith In The Shadow Empire 

In the days of old, there was once many Dark Lord's of the Sith or Dark Lady's of the Sith. In the Rule of Two, during Darth Bane's Sith Empire, there was only a Dark Lord of the Sith and a Sith Apprentice. As the Rule of Two, it dawned to be known as one of the greatest strategies in Sith history, such as Dark Lord Darth Sidious and his Sith Apprentice Darth Vader. The only way that Sith Apprentice, Darth Sidious was able to go from the rank of Sith Apprentice to Dark Lord of the Sith, was to kill his own Sith Master, Darth Plagueis.

In the Rule of Two, you can only have one Dark Lord of the Sith and one Sith Apprentice, but with the uprising of the Shadow Empire, Dark Lord of the Sith Onepath Rainwar re-created the Dark Council, a Council just like the Jedi High Council. In the Dark Council, there could be twelve Dark Lords of the Sith at once like there can be twelve Jedi Masters on the Jedi High Council. With the uprising of the Shadow Empire, there's only one spot filled in the Dark Council.
 * Dark Lord of the Sith Onepath Rainwar