The Emperor had foreseen Vader killing him during a large battle. So the Emperor formed an elite group within the Crimson Guard known as the Blood Shadows. They were formed to destroy Vader's elite Stormtrooper corps in the event of a coup to overthrow the Empire. They were also the Emperor's enforcers, punishing villages who harbor rebel operatives. They not only murdered the inhabitants, but destroyed the entire village, without leaving a trace of who was responsible . They were kept secret from Vader and the Imperial Senate. If the Imperial Senate knew the atrocities the blood shadows commited, there would be a unanimous removal of Palpatine.
The inspiration for these guys was the Dark Troopers from Battlefront Renegade Squadron, i was pretty beast with those guys. I gave one a jump pack. Another is just a commander.