1/ To get my mind off things, here's some Star Wars lore for ya - How the Jedi knew about Darth Bane and his Rule of Two (per Yoda's line in TPM):
Around 990 BBY (10 years after the New Sith Wars had ended and the Ruusan Reformation occurred), Darth Zannah (along with her

2/ cousin, Darovit), wanting to heal her master, Darth Bane, from a deadly poison and his orbalisk armour, sought the help of a healer, Caleb. Caleb refused to heal Bane unless Zannah sent a message to the Jedi Temple on Coruscant to turn themselves in. After doing so and

3/ informing the Jedi, Caleb healed Bane. Zannah then killed Caleb and drove Darovit insane and gave him the lightsaber of a Jedi Master that was killed during the duel on Tython. Knowing the existence of this surviving Sith Lord Bane and his apprentice, the Jedi team

4/ arrived and were attacked by the deranged Darovit. Believing him to be the Sith Lord in the message they received about a surviving Sith, the Jedi struck him down and believed that he was the Sith Lord called "Bane". While this happened, Zannah hid herself and Bane. The Jedi,

5/ believing they defeated the last of the shortly-lived revived Sith, retreated back to Coruscant.
Cut to 800 years later (circa. 190-80 BBY) when Jedi Padawan Kibh Jeen and his master travelled to the planet Almas, which was home to a long-abandoned Sith Fortress. Driven mad by

6/ the strong dark side presence, Jeen succumbed to the dark side and cut down his master in a fit of fury. Over the next several years he had become a Dark Jedi and learned of the Banite Sith and the new Rule of Two tenet set forth by the last Sith, Bane. When the Council had

7/ decided to send a Jedi and her Padawan after several years of the mad Jeen waging conflicts within the system, they had cut down the fallen Jedi. his dying words were about how "there were always two Sith, no more, no less." Believing the words to be nothing but the rants of

8/ a madman, the Jedi Order dismissed it, until years later when Darth Maul revealed himself and realized Jeen's words more than a century earlier were a warning and that the Sith had indeed returned without them knowing after all these years. #StarWars #IChooseLegends

9/ (sources: Darth Bane: Rule of Two, Jedi vs. Sith: The Essential Guide to the Force, The Phantom Menace)