Shadow Hand
From Wookieepedia, the Star Wars wiki.
Shadow Hand was the title given to the second-in-command of the Sith monarch, 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 an apprentice.
Adas' Shadow Hand would inherit 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.
