The Cabal of the Hidden Hand was a secret order of Force-sensitive assassins.
The Hidden Hand was initially formed during the New Sith Wars by a group of rogue Jedi Sentinels (whose ideologies would later inspire the mysterious Jedi shadows) that wished to utilize Gray Jedi-style mastery of the Force in their wars with the Sith. Following the war, the rogues remained banished from the Order and began offering their assassin skills to corrupt governors and warlords across the galaxy, while also remaining true to their vow to eliminate dark siders, Sith or not, whereever possible. These assassins began recruiting other Force sensitives through secretive and elaborate methods, allowing the organization to survive for centuries. Many of these recruits over the years were rogue Jedi. A complex holocron known as the Hidden Chronicle was created by one of the founders (in the Late Middle Basic language) detailing the group's methods, tenets, and philosophies; this holocron later ended up on the black market after it was captured by the Mandalorian warrior Naastau during his forays into treasure hunting.
At some point, the Hidden Hand began serving the Black Sun crime syndicate, often playing a role in the Black Sun's wars with the Hutts. The Jedi Order was aware of their existence and history, however was unaware that the bulk of their numbers were still Force-sensitive and thus regarded them as a common mercenary gang. Some of these assassins fought in the Clone Wars as acolytes of Darth Tyrannus.
During the Galactic Civil War, the Hidden Hand was led by Stizicar Yelmos, who was a former Dark Acolyte and a personal agent of Prince Xizor. He was killed by a Jedi Knight in 1 ABY in his "temple" on Ord Mantell.
While the first Jedi who founded the Hidden Hand wielded lightsabers in their battles, future generations typically wielded vibroblades and blaster pistols. These assassins also made use of various black market armor as well as gadgets like shield generators and cloaking devices. Each member of the Hidden Hand usually owned their own starcraft, usually a starfighter or gunship, that was also often equipped with cloaking technology.