Shadow Hand is the title given to an apprentice of the Dark Lord of the Sith. The Deputy Grand Master is often a Shadow Hand.
A Shadow Hand is considered an apprentice 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 Darth.
Shadow Hand was the title given to the second-in-command of the Sith King, or Sith'ari. 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.