With all credit going to Azronger:
Originally posted by Azronger
For a millennium, the Sith maintained the order in secrecy, passing down their evil heritage. As they gained knowledge of the dark side of the Force, their powers increased with each generation.-Episode 1: The Phantom Menace Scrapbook
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Ultimately, Bane's plan produced more powerful Sith Lords with every generation.
-Force and Destiny
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For a thousand years we continued to follow Bane's Rule of Two, existing in the shadows, biding our time, growing in power, feeding our hatred.
-Insider 88: Heritage of the Sith
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"Bane's power has been passed down for a thousand years. I vow to be its last recipient."
-Darth Sidious, Book of Sith: Secrets from the Dark Side
Concession accepted.