This forgotten ancient Sith Lord explains Palpatine’s rise and fall
One shadowy figure changed the Sith's entire future.
There are a few universal truths in the Star Wars universe. Han Solo never wants to know the odds, cantinas will always be full of colorful characters, and a Sith will always fall at the hands of his apprentice. This was true of Vader and Palpatine, of Palpatine and his master, Darth Plagueis, of Plagueis and his master, Darth Tenebrous, and so on throughout the ages. It wasn't always this way, though. One Sith established this rule, and in doing so, shaped the Star Wars universe as we know it.
Darth Bane has always been waiting in the wings of Star Wars canon. He was born a millennium before the events of the original trilogy, but was an important character nonetheless. In the non-canon Star Wars works, its was established he was a young Sith upstart who got disillusioned with the Sith leadership and forged his own path, similar to the heel-turn Anakin would take with the Jedi much later.
Bane overthrew the Sith Brotherhood, the Sith governing body, and established the Sith Order that would remain everafter. Fed up with all the ambitious infighting, he established the Rule of Two: there can only be two Sith at any one point, and the master will always die at the hand of the apprentice.
Darth Bane doesn't only exist in non-canon works, however. He also appeared in The Clone Wars. He was first set to appear during The Mortis Arc, which explored the intricacies of the Light and Dark sides of the Force.
Unfortunately, his parts ended up on the cutting room floor, but this allowed for him to be incorporated in "Sacrifices," the Season 6 finale, appearing to Yoda as a spectral vision to convince him to turn to the Dark Side.
Curiously, he was also voiced by Mark Hamill, so the face of the Star Wars franchise lent his voice to the founder of the epitome of Star Wars evil.
With that appearance, Bane, his backstory, his rebellion, and his eventual murder by his apprentice, Darth Zannah, are considered canon. Another appearance in a future series or movie is possible. After all, Order 66 did not as much complete Palpatine's plan for personal power as much as it completed Darth Bane's centuries-long search for it. He's the founder of the Sith Order, and that's an important part of lore for any fan, and deserves to be told in its own way.
With the Skywalker Saga over and done with, this is the perfect time for Star Wars to explore its roots and make a prequel to the prequels, set hundreds of years before the events we've seen on screen before. For a franchise that needs a new path for the future, maybe the answers are in the past.