The Sith assassins were a sect of covert Force-sensitive killers secretly founded by Darth Revan on Malachor V during the Mandalorian Wars. Captured Jedi and Sith soldiers who were found to be Force-sensitive were taken to the Trayus Academy to be trained as Sith assassins.
The Sith assassins were unique because they preferred to ambush their intended prey from the shadows rather than engaging them in open combat. Almost always working solo or in small groups, they utilized stealth field generators and the rare art of Force camouflage to aid their surprise attacks. While some elite assassins fought with lightsabers, the majority utilized less remarkable melee weapons, such as force pikes. These stealth warriors were also unusual in that they drew their power from the Force presence of their prey. The stronger their targets were in the Force, the stronger the hunters became.
During and after the Jedi Civil War, the assassins' primary purpose was to kill or capture Jedi. Upon successfully incapacitating a Jedi, the captive would be tortured until they fell to the dark side or died by the hands of elite Sith operatives trained in the art of torture and temptation, such as Atton Rand. By this subtle method of capture and conversion, the ranks of the Sith assassins swelled as the number of Jedi decreased.
The Sith Assassins reached their zenith under the leadership of Darth Traya, Darth Sion, and Darth Nihilus following the almost complete annihilation of the Sith Empire's Dark Jedi during the Sith Civil War. Based within the hidden Trayus Academy on Malachor V, the Sith Lords commanded legions of Assassins to hunt down the remaining Jedi throughout the galaxy. Their efforts proved quite successful as they very nearly caused the complete extermination of the Jedi Order.
During the New Sith Wars, the Brotherhood of Darkness trained Sith assassins in the Ryloth Academy, Umbara Academy, and Nar Shaddaa Academy. At least eight of the Umbaran Sith assassins survived the New Sith Wars, and joined Hetton's Anti-Republic Liberation Front. However, they were all killed by Darth Bane during a duel when they were brought to Ambria by Darth Zannah. The One Sith incorporated an agency called Sith Intelligence and Assassination.
In Star Wars: Knights of the Old Republic II: The Sith Lords, Sith assassin is one of the six prestige character classes which the main character can select after reaching level 15. Like its light side counterpart, Jedi Watchman, this class corresponds to the Jedi Sentinel non-prestige class. All the in-game Sith assassins have this character class. Jedi Watchman and Sith Assassin, unlike the Jedi Master/Sith Lord and Jedi Weapon Master/Sith Marauder counterparts, are not "one for one" ability trades. Sith Assassin has a lower feat and save progression, but gains more sneak attack dice.
When you enter the Trayus Academy, an assassin appears with two lightsabers; a normal one, and a short-bladed. This is the one of the two elite Sith assassins. An elite Sith assassin appears on Citadel Station wielding a double-bladed lightsaber during the battle of Telos IV in KotOR II.
The Sith Assassins in KotOR II wear masks that have striking aesthetic similarities to the mask worn by Darth Vader (without the helmet).