Shadow Guardsmen are the Emperors personal agents within the Royal Guard. These agents are often directed to eliminate a jedi survivors from the clone wars or newer force sensitives not under Imperial control. Shadow Guards were chosen from the ranks of the Royal Guard due to their force sensitivity.
They were given training in the ways of the dark side along with standard combat and intel training given to Palpatines agents. Their are unconfimred rumors that some Shadow Guards were surviving jedi who were brainwashed and drugged into Imperial service
The roots for the organization start near the end of the clone wars when Count Dooku ordered Asajj Ventrus to eliminate the Sun Guard. The surviving members went on to join the Royal Guard and any force sensitives went to joint the Shadow Guard.
in 17BBY 2 members of the Shadow Guard sent to a Golan space platform in orbit of the planet Byss. They served the Imperial Inquisitors in a training capacity until an Aldereenian resitance movment attacked the station. The Shadow Guards were able to fight and kill numerous resistance fighters
Shadow Guards were most notoriusly seen in the year 2BBY during the hunt for Galen Marek. A guardsmen tried to eliminate the former Secret Apprentice on Nar Shaddaa while commanding a regiment of Shadow Troopers and Imperial Navy Commandos. They were slaughtered by their target. A lone Shadow Guard accompanied the regiment of Stormtroopers on Cloud City hunting for Marek. This guardsmens was electrocuted to death via force lightning while most if not all the troopers were killed as well.
A another shadow guard attack was revealed to be PROXY using the Shadow Guard program he had stored against his master. The final Shadow Guard sighting that year was outside of Emperor Palpatines chamber as a last line of defense. This guardsmes leada group of Senate Guards on a final assualt on the rogue apprentice and failed.
The possible last known sightining was in 1BBY when a large number were seeing guarding the cloning facilities on Kamino. These guards were wiped out during the Battle of Kamino during the Galatic Civil War.
Since the Shadow Guards were taught about the darkside they were able to call upon the force and use its power. They were taught the more offensive powers of Force Choke, Force Malestrom, Force Push, Force Repusle, and even Force Lightning which Darth Vader could not due.
Shadow Guards main weapon was a lightsaber pike with a red blade derived from a synthetic crystal. For medium range combat they wield heavy blaster pistols. The black armor was based on Sun Guard armor and was more than cerimonial as it provided ample protection while allowing movement for melee fighting. All guardsmen were equipped with medipacs and longe range personal comlinks with holographic capabilities.