The Origins Of The Rivalry Between Jedi And Sith
To many, the story of Star Wars is the ancient battle between light and dark, good vs evil, Jedi vs Sith. However what exactly started this great divide between these two sects of force wielding people? Well. the feud began in something that is related to both canon and Legends, and this is a great event called the Hundred Year Darkness.
The Hundred Year Darkness is the official split, when a group of Force-sensitives broke with the Jedi order and embraced the dark side. However in Legends, the event is detailed in more depth. So from here on out, we will mostly consider that interpretation of events. However, at its core the event remains the same. A bunch of Jedi split off and embraced the dark side, and thus a war broke out. The Dark Jedi were defeated and exiled until they found a planet called Korriban and a species called the Sith.
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These dark siders thought that the Jedi were limiting their knowledge and ability to command the Force. While not the first schism in the order, it was certainly the most influential, led by a Sith in canon by the name of Adjunta Paul. War broke out. However, at the end of the day, the Jedi outnumbered these dark perversions of themselves, as seems to be the way with many of these Sith wars.
On Korriban, the Dark Jedi dominated the Sith species becoming the first Dark Lords of the Sith. Many centuries passed as those Dark Lords built a mighty empire far away from both the Jedi. It lasted for a thousand years, and there was lots of breeding between the Dark Lords and the Sith, creating hybrid species whose ancestors go back all the way to the original dark side exiles who once split off from the Jedi.
Not until the era of Naga Sadow did the Sith Empire reach the height of its power and influence. He launched an attack on the Republic and the Jedi almost attacked Coruscant itself. However, the Sith were repelled, and the Republic attacked. They went overboard, though. Republic forces, with the aid of the Jedi, wiped out the Sith species completely...or so they thought. For the true Sith went into exile in the Unknown Regions and began to rebuild their Empire under the command of one of the survivors of the Republic onslaught, by the name of Vitiate.
Though that was not the end of the Sith, for their ghosts and spirits would continue to operate from the shadows, looking for fallen Jedi to teach and turn them into Sith Lords who would continue their work in the dark side of the force. From the likes of Freedan Nadd to Exar Kun and eventually Revan, these fallen Jedi would continue the ancient rivalry of the Sith, passing and growing the knowledge of the dark side all the way to the farthest future of the Star Wars timeline. Though different in personality and goal, all the Sith carried a burning hatred of the Jedi as their sworn enemy, and most loathsome of foes.
Now again, most of this information comes from Legends. In canon, all we know is that an event called the Hundred Year Darkness occurred, thus splitting the Jedi and the Sith into two separate but powerful factions of Force users whose rivalry would stretch for thousands of years. Of course, we can infer that some of the same events still happened more or less the same, for the Sith did find the planet of Korriban (now called Moraband) and built mighty empires that fought the Jedi and the Republic, thus beginning the never-ending struggle between the Sith and the Jedi.
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Source(s): Wookieepedia