Sullust is the main planet of the Sullust system, forming the conjunction of two major trade routes. It is a very unstable planet with hundreds of active volcanoes, a toxic atmosphere, and intense electrical storms. The native species of the planet, for these reasons--and because of the vast underground mineral deposits--live in underground caves, where there is a diverse ecosystem supported by vast underground lakes. The planet is most well known for being the headquarters of the SoroSuub Corporation.
The planet was governed by the Sullustan Council, the ruling body of Sullust. Originally elected just by the Sullustans, the Council became more and more under the control of the SoroSuub Corporation, as their shareholders gained the ability to vote in elections for the Council, even if they weren't Sullustans or even on Sullust. By the begenning of the Clone Wars, the positions of CEO of SoroSuub and President of the Council were held by the same individual. During the Clone Wars, the planet seceded in Republic sometime during 22 BBY, joining the CIS.
The ruling council of the planet was eliminated when SoroSuub--and ergo Sullust itself--declared its allgiance to the Empire, under Proclamation 173.D, which explicitly stated that SoroSuub had complete control of the planet.
This angered many of the inhabitants of the planets, and during the Galactic Civil War, several resistance cells formed. The leader of the resistance cells, a Sian Tevv, eventually gained enough popularity and power to take control of Sollust away from SoroSuub just before 0 BBY. Although the resistance cells were dedicated to freeing their own planet and were not part of the Alliance to Restore the Republic, they did allow the Alliance fleet to use their planet as a staging point for the destruction of the Death Star II.
During the First Battle of Sullust, Rogue Squadron flew Y-Wings to destroy an Imperial installation hidden within a volcano on Sullust. However, it turned out that it was a ruse to draw Rogue Squadron away from Thyferra in the Battle of Thyferra.
The Second Battle of Sullust was an effort to draw the mainstay of the Imperial fleet away from Endor. Although Vader seemed concerned about the presence of the Rebels, Emperor Palpatine did not, and paid them little attention.
During 6 ABY, Emperor Bacharan Valak annexed Sullust and put the inhabitants of the planet to work in slave labor camps, where they were treated very harshly. This prompted the New Republic to liberate the planet again, although they were too late to have any real effect on saving the population--upon arrival, they found the corpses of millions of Sullustans who had been killed during the Imperial Occupation.
During the final stage of the Imperial Blitzkrieg, Danik Kreldin launched a siege of the planet, which was one of the last remaining Republic strongholds, and would finalize cutting Coruscant off from the main trade routes of the New Republic. Although the fighting was heavy, as were the casualties on both sides, the planet eventually fell into Imperial hands.