Legacy begins just over one hundred years after the end of the Yuuzhan Vong War. The Jedi Order has allowed the shaper caste of the displaced Vong to terraform a desolate portion of the planet Ossus, hoping to restore the barren world to life and redeem the invaders to the galaxy at large. Known as the Ossus Project, it was championed by Kol Skywalker, one of the leading members of the Jedi Council.
Since the defeat of the Yuuzhan Vong, the Imperial Remnant has built up its resources and forged itself into a more benevolent Fel Empire, with a more benign Emperor, the third of the Fel dynasty, on the throne.
The success of the Ossus Project inspired other planets to vie for the opportunity to be terraformed, and the results were briefly a success, before the process began to devastate ecosystems, and cause painful bony growths to spout from the inhabitants’ skin. Still distrustful of the Yuuzhan Vong, the galaxy knew who to blame. The Jedi suspected sabotage, and convinced the Galactic Federation of Free Alliances to side with them. The Moffs of the Empire, however, invoked the Treaty of Anaxes, and declared war on the Galactic Alliance.
Those responsible for the sabotage were in fact a new Sith Order, which had been secretly building up its strength on the planet Korriban for over one hundred years. Two years into what would become known as the Sith–Imperial War, the Sith, and their leader, Darth Krayt, would reveal themselves to the galaxy and join the war on the Empire’s side. After three years of bloody fighting, the Galactic Alliance was forced to surrender. Though they were officially absorbed into the Empire, a remnant fleet broke away, under the leadership of Admiral Gar Stazi. The Jedi Order retreated to Ossus, only to be followed there by the Sith..
Fuente: Star Wars Wikia