The Black Sun was founded some time between the years 3653 BBY and 3644 BBY in the aftermath of the Great Galactic War. Following the war's end and the sacking of Coruscant at the hands of the Sith Empire, Galactic City soon descended into chaos and anarchy with rioting and looting becoming normal and the Coruscant Security Force was already streched thin in the aftermath of the devestating attack.
Because of this chaos, various gangsters got together in the lower neighborhoods and began siezing the lower levels for themselves. The name Black Sun would show up in sayings such as "better a black sun than none" in reference to the lighting in the lower levels of the planet and thus, the Black Sun crime synidcate was officially born under the name. In little time, the Black Sun quickjly expanded and began claiming territory for themselves in the lower levels and used it as a base of operations for their new organization.
Their advancements wouldn't go unoticed and they were soon haulted a Justicar's Bridage, but the rioting and looting within their own territory had not ceased regardeless. In the following years during the Cold War, the influence of the Black Sun had only continued to grow and they had eventually expanded beyond Coruscant itself and began spreading across the galaxy.
As the organization expanded, the Galactic Senate soon found out about their existance and the threat they posed after reports came in of CSF officers of the Special Tactical Unit were seen fighting Black Sun members in an attempt to regain lost territory siezed by them earlier on.
The Clone Wars
Followignt he outbreak of the Clone Wars in 22 BBY, the galaxy soon found itself once again split apart and divided between the two governments, the Galactic Republic and the Confederacy of Independent Systems just like it was during the Old Republic era between the Republic and the Sith Empire.
The Black Sun had taken full advantage of the conflict to once again expand and grow their influence across the galaxy at a time when the Republic and Jedi were too busy dealing with the Separatists. During this time, the Falleen species rose to prominance as the dominate race that members of the Black Sun were and the organization was effectively under the control of a Falleen cabal sometime during the war.
During the later stages of the conflict, the renegade Sith Lord Darth Maul, rogue Sith apprentice Savage Opress, and Death Watch leader Pre Vizsla traveled to the Black Sun fortress on Mustafar where the three of them approached ruling council to make them an offer they couldn't refuse. They turned it down and Savage responded by killing the entire council except for one Falleen male, Ziton Moj, who took over as the Black Sun's leader and aligned the organization with Maul and the Death Watch, leading to the creation of the Shadow Collective.