Dark Lord of the Sith is the formal title taken by the Grand Master as the Sith Master of the Order of Sith Lords.
While holding the title of Dark Lord of the Sith the Grand Master may style himself as either Lord or Darth. Upon retirement, a Grand Master loses their position as Dark Lord and become an ordinary Sith Lord.
The current Dark Lord of the Sith is Lord Dantes.
Lord of the Star Chamber is the title taken by the ruler of the Star Chamber of Antei.
The title originated in the personage of the Sith King of Antei, Okemi, the original Lord of the Star Chamber following the Great Sith War. Today the title is claimed by the Grand Master of the Dark Brotherhood.
History
The title of Lord of the Star Chamber formally originated c. 3950 BBY when Okemi came together with the Mecrosa assassin Ferran and the Krath sorceress Taimat to found the Star Chamber.
However the Star Chamber has its origins much earlier. Okemi was a Sith warlord who was cast out of the Sith Empire by Marka Ragnos in c. 5050 BBY. Fuelled with hatred for the Dark Lord of the Sith, Okemi established himself as King of a remote planet known as Antei. Later, with the arrival of the two who would one day become the Star Chamber's ruling triumvirate Okemi took the title Lord of the Star Chamber in direct opposition to the Dark Lords of the Sith.
Modern usage
The title of Lord of the Star Chamber was first revived by the Dark Brotherhood in 13 ABY following the excavation of an Obelisk temple. Following the crisis with The Other, Jac Cotelin resurrected concept of a ruling triumvirate to prevent one man ever endangering the Brotherhood again. The title itself was taken to honor the Sith King Okemi mentioned in tablets discovered at the Obelisk ruins.
The Brotherhood's usage of the title would become genuine after the Great Exodus. It was not until the Fifth Great Jedi War that the true nature of the title became known when the arrival of the Dark Lord of the Sith on Antei awoke the vengeful spirit of Okemi, long since driven mad by the Nameless Ones. Following Okemi's death Lord Firefox reclaimed the title of Lord of the Star Chamber, finally unifying the two ancient orders.