shadow guards were used as assassins. they would hunt down suspected jedi. it was possible they were brainwashed jedi, but it was never proven. they could use abilities such as Force lightning, Force Repulse, Force Push, Force Choke and Force Maelstrom. force-sensitive troopers from the sun guard may have joined the shadow guards, but it was never proven right.
At 17 BBY, two guards were stationed at a Golan space defense platform near the planet Byss. They were attacked by Alderaanian rebels and fought them off.
A single Shadow Guard and seven Shadow Troopers were sent to Nar Shaadda when Galen Marek tried to find Jedi Master Rahm Kota in a local bar called The Undertow. The Guard may have survived Galen Marek's Force Maelstrom, but most people think he died.
Four Guards went to Cloud City on Bespin to kill Galen Marek, but only one found Galen Marek. This Guard was hit by his own force lightning and Galen Marek's at once, killing him.
one was stationed on Kashyyyk to stop Galen Marek from reaching the Imperial Skyhook, but Galen Marek ran him through with his own Lightsaber Pike.
A Shadow Guard was found on Raxus Prime in the Imperial Ore Facility, though this was actually PROXY using a program based on a Shadow Guard to try to kill Galen Marek. Later, Galen Marek actually encountered a real Shadow Guard at the Ore Cannon Control. He met his end when Galen Marek trapped him in a Force Grip and threw him into the ore cannon, incinerating him.
A final known Shadow Guard led the last line of defense of Palpatine's watchtower on the Death Star I, this one backed by a quartet of Royal Guards wielding lightsaber pikes. The guard was grappled by Galen Marek, who broke his neck and killed him.