Shadow Hand is the formal title taken by the Deputy Grand Master as the apprentice of the Dark Lord of the Sith.
A Shadow Hand is considered a member of the Order of Sith Lords. After completing their training a Shadow Hand will become a Sith Lord and be able to assume the title of Lord or Darth.
The current Shadow Hand is Jedgar O. Paladin.
List of Former and Present Shadow Hands of the Dark Brotherhood
History
Shadow Hand was the title given to the second-in-command of the Sith King, or Sith'ari, of the Old Sith Empire before the rise of the Dark Lords of the Sith. Later, members of the Sith Order would use it as both a title for the second-in-command of the Sith Master and as a term equivalent to a Sith Apprentice.
Adas's Shadow Hand inherited the fallen King's holocron, while another Shadow Hand, many thousands of years later, betrayed his master to the newly arrived Dark Jedi, thus paving the way for the first of the Dark Lords of the Sith.
The concept continued with later Sith. Darth Vader trained his own disciples, among them Lumiya and Flint, in the hopes of pitting them against one another to find a champion worthy of being the Dark Lord's own Shadow Hand.
The reborn Darth Sidious used the name for his campaign to retake the Galaxy.