A knack of doing the impossible was what Sith Grand Admirals were selected for.
—Unidentified smuggler
Grand Admiral, was a rank in the Imperial Navy of the resurgent Sith Empire. Bearers of the rank were some of the Empire's top commanders, appointed by the Sith Emperor himself. Only eleven Grand Admirals served the Empire, appointed in a ceremony in 3,659 BBY, during the Great Galactic War.
The rank was also used by Darth Revan's Sith Empire. The naval commander Hasek Grunger was promoted to the rank after his conquests and displays of tactical knowledge. However, the exact position of the rank in the Sith Navy's hierarchy was unknown. Interestingly, Grand Admiral Grunger appeared to be a subordinate of regular admirals.