Star Wars: The First Sith Explained
The Hundred-Year Darkness
Around 7000 years before the Battle of Yavin, there came a period known as the Hundred-Year Darkness. Cracks started to appear in the Jedi Order when a section of the Knights started experimenting with the Force and how it could be used, pushing the boundaries further than ever before. These rogue Jedi discovered that they could harness the Force to bend life itself and create new species, and that only by tapping into the Dark Side could true mastery be achieved.
Obviously, the Council were against such practices, and the rogue Jedi were cast out into exile, thus beginning a war that would last for one hundred years. These rogues were led by Ajunta Pall, a member of the Jedi Order who rose to the rank of Master, before his studies of alchemy took him to the Dark Side.
Numerous conflicts were fought across the century, with the Dark Jedi constantly being pushed back by the Light, and coming back with new, even more fearsome creations each time, only to suffer further defeat.
Eventually, after defeat on the planet Corbos, despite Pall himself slaying a dozen Jedi during the conflict, the Dark Jedi were rounded up by the Republic and sent into hyperspace.
The Exiles eventually found their way to the planet Korriban, where lived a species known as the Sith. These humanoid creatures had bone spores and tentacles on their red faces, and were highly-attuned to the Dark Side of the Force.
Led by King Hakagram Graush, the Sith initially resisted tothe efforts of the exiled Jedi to settle on their homeworld, but were finally won over by their advanced technology and clear proliferation in using the Force. The Dark Jedi Pall managed to collude with Graush's Shadow Hand and eventually slay the King himself, with the Dark Jedi worshipped as a God-like species on the planet.
Pall, then, assumed the title of the first Dark Lord of the Sith, with his followers interbreeding with the Sith race, thus creating an Empire that he'd expand into new worlds, and would span centuries and fight against the Jedi Order time and time again.