1,000 BBY

Darth Bane: Path of Destruction - Part Three
(Del Rey Books - Oct 2006)
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Jedi vs. Sith
(#1-6 - Dark Horse Comics - April 2001)
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The Sith turn against one another and nearly all are destroyed. One of the few survivors include Sith Lord Kaan, who gathers his army for a final conflict with the Jedi.

The Battle of Ruusan. Thousands of Jedi die in a great battle on the planet Ruusan between two Jedi armies -- Lord Hoth's Army of Light and Sith Lord Kaan's Brotherhood of Darkness. Kaan refuses to surrender and creates a weapon using the spirits of the Dark Jedi. The weapon kills the Jedi and Sith, but traps their spirits within the caverns later known as The Valley of the Jedi. They will remain trapped for over a thousand years before they are freed by Kyle Katarn. The battle signals the end of the millenium-long New Sith Wars.
Jedi Knight: Dark Forces; The Essential Chronology; Star Wars Insider #80: "The History of the Mandalorians".
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Darth Bane: Rule of Two - Chapters 1-9
(Del Rey Books - Dec 2007)
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"Bane of the Sith"
(Star Wars Gamer #3 - Wizards of the Coast - Apr 2001)
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Darth Bane, the single Sith survivor of the battle at Ruusan reinvents the Sith order. He decides that there can be only two Sith at one time: a master and an apprentice. The Jedi Knights believe the Sith are destroyed, but this new order secretly survives for the next millenium.
The Essential Chronology; The Phantom Menace novel.
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The bounty hunter Durge, who had worked for the Sith, goes into hiding, fearing Jedi reprisal.
Starwars.com Databank.
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The military tactic known as the Erasmus gambit is first used.
Rebel Alliance Sourcebook, via Time Tales.
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The GenoHaradan assassins' guild establish the Tac-Spec Corporation to ease the sale of  its Footman droids.
The New Essential Guide to Droids.
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In the aftermath of the New Sith Wars, Supreme Chancellor Tarsus Valorum inacts the Ruusan Reforms, giving more power to the Senate and formalizing the Jedi Order's ties to the Republic government.
The New Essential Chronology.