The Solari Sith were founded by Darth Solari, an Anzati whom turned his back on the Sith Empire after it collapsed again in 1,010 BBY when it became close to complete dominance over the galaxy. He founded his own Sith sect based on his own interpretation of the Rule of Two, resulting in several powerful darksiders that became key players in the rise of the Sith Order of Darth Sidious. The group eventually became a useless pawn, and was folded into the One Sith.
Darth Solari was born on Anzat in 1,040 BBY, recruited by Sith Kaan as one of his personal tools, codenamed Solari by Kaan. He supported the Sith Empire as it rampaged through the galaxy, the Republic reduced to just the Core. Coming of age as a Marauder, he came into the centre of the Sith Empire's government as it focused solely on Ruusan and subsequently lost everything - Solari departing from the system after a year of pointless fighting there.
Disillusioned by the Empire and Kaan training numerous apprentices, Solari annointed himself Darth Solari and sought to recreate a two Sith system - drawing upon Darth Rivan as his inspiration - where Darth Solari would be the Dark Lord and his apprentice would be moulded into an equal. This resulted in him meeting Set Harth, who proved too much of an equal, and Solari was forced to spend centuries not only hunting down Brotherhood of Darkness remnants and his first apprentice.
Considering his first effort a failure, Darth Solari focused on a different approach, seeking to create groups of darksiders that were no threat to himself but could create a pool that he could draw upon. Instead, he modified the Sith Rule of Two into a Rule of One - his Rule - aware he could live for a thousand years with ease if he took care of himself. Those efforts were fruitless as it became more difficult for him to discover new Force users without attracting the ire of the Jedi. Solari circa 600 BBY realised this danger could be a boon; he could use the Jedi as his pawns.
In 600 BBY he coaxed Allya from the Jedi Order and vaguely directed her towards Dathomir. The Nightsisters would eventually represent a pool of darksiders he could reach into whenever he required them. He knew they would never become truly powerful or dangerous, however, - and nor would he let them - and sought to make efforts to create a darksider he could use as a tool.
It was with this in mind he returned to Coruscant and molded Volfe Karkko into a powerful Sith Lord, enticing him from the Jedi order. Annointing him a Sith, the links of his apprentice to the Jedi Order eventually drew the eye of the Jedi Council, who imprisoned the Sith. Decades of work ruined, Solari began anew, making efforts to restart the process - he had no need for a failure, and releasing the apprentice would only expose himself.
While the Nightsisters represented a relative success, it wasn't until Darth Solari discovered Aurra Sing, a cast-off Jedi and bitter Padawan, that Solari felt he had a success he could be proud of. Taking her to Anzat, he judged her no psychological threat to him - she wasn't interested in power and galactic dominion, only revenge - and he made efforts to train her, never revealing his identity to her. Pleased with the tool he had created, he unleashed her in the underworld of Coruscant, several Jedi being slaughtered by her and the Jedi being mired in their own links to the ex-Padawan. The strategy, of psychologically pathetic but dangerously powerful pawns, seemed to have paid off, and Solari made efforts to groom another target.
To solidify his efforts, he placed a Nightsister among the Black Sun, and prepared to co-opt the organision into a strike against the Republic - with Aurra Sing running rogue, Solari had a perfect distraction should he require it, allowing him to prepare an audacious strike on the Republic, accelerating the education of several Nightsisters to make them more potent pawns.
The Stark Hyperspace War and Yinchorri Crisis passed, Solari paying them no heed, completely failing to notice the undercurrent perpetuating the rot in the Republic. He quickly had everything he needed, preparing a Black Sun-supported coup on Coruscant and the deployment of the Nightsisters to key positions across the Republic.
As quickly as it finally came together, everything fell apart. In 32 BBY Black Sun was decimated, and Solari frantically backpedalled, trying to find where this danger was striking at him from. Before he could detect the danger, the Sith emerged from hiding and Darth Maul was left as evidence to him, evidence Solari could not ignore.
His plans began to take on a life of their own, the Jedi putting down an effort by his Nightsisters to destroy Coruscant. Solari noted Quinlan Vos as a element whom was responsible for the death of Karkko - no real loss - but suddenly found himself with little to work with. However, Solari had no doubt he could use the Nightsisters again, when they had recovered on their own. Solari watched as Aurra Sing continued to be a thorn in the Jedi, and moved onto isolating Ky Narec from the Jedi, and preparing to manipulate him into a new Sing.
Equipping Rattataki warlords with the weapons to ground Narec and kill him - he had no need of a useless pawn - and was surprised to discover Asajj Ventress. By this point, the fires of the galaxy were giving birth to a Separatist movement, and Solari found his pawn co-opted by Count Dooku. His Nightsisters were being corrupted into Tyranus' pawns, and establishing their own motives; Solari was entirely out of the loop.
Ruined, he fled to the Stenax, seeking somewhere to hide, before these dangerously active and seemingly powerful Sith noticed him. He eventually found himself an apprentice, Darth Khopesh, and began forging the Stenax into tools to massacre humans. He knew, with enough time, he could forge them into a galactic power. If he chose the right moment, he could break the Empire.
The Battle of Endor came about, and the Empire suddenly collapsed. The Stenax worked themselves into a rage and, without Solari's direction, struck, launching the Stenax Massacres, which were quickly put down by the Republic. Solari, reaching the age of 1048 and demoralised, left for the Unknown Regions; he was nothing, and had wasted his life.
By 128 ABY the One Sith emerged from hiding, and brought with them a legion of Sith. Solari knew, in his heart of hearts, that the effort was doomed to complete failure, but he wanted to live the Sith dream of domination, to conquer the galaxy with a Sith Empire. He joined the Sith Empire, and assisted them in capture of Ralltiir. He had little interest in the war they were playing, just to play his part in the destruction.
As such, the irony that he was killed by Darth Riven - the One Sith replacement for Darth Rivan - was amusing to him. He drained the bloodsoup of the young Sith before his death, but died at Ralltiir nonetheless.