Darth Bane, born Dessel, also known as Goldust, Dustin Rhodes, Big Guy (for you), Black Reign, and Ziggy Stardust, was a rugged individualist Sith with a penchant for gold skin-glitter and eye makeup. He created both the Rule of Two and the Rule of Stew.
He was born in 1026 BBY, which was a very dull time in the history of the Republic. In fact, Bane's arrival on the scene was just about the only interesting thing that happened that millennium. He was born to an abusive drunken father on Apatros and spent his youth among sweaty, well-muscled men slaving away in close quarters in the cortosis mine. It was an unconventional childhood, but at least he grew up okay. Oh, by the way, he killed his dad in a fit of rage using Force crush.
"Don't know much about villainy Don't know much about depravity Don't know much of how to scheme and plot Never won a single duel I've fought..."
―Darth Bane, singing the blues at school
Dessel escaped, spent a stint in the army, killed a bunch of people, then got recruited by the Brotherhood of Darkness, a kind of "Sith lite" organization that had been waging war against the Republic for a few decades. Not having much luck. He trained on Korriban, where he was hailed as a promising new student, maybe even the long-waited Sith'ari... but somehow was always getting his ass handed to him in lightsaber duels. Humiliated, he went to an expensive Japanese juku and crammed for his lightsaber exam. He fell in love with his teacher Githany, which didn't help his concentration much... but then Bane discovered that his Force lightning abilities became much more powerful after a week of Githany withholding sex. This trick served him well in the years to come.
Bane kept failing at everything he tried, so he flounced out of the Brotherhood headquarters and went to the Valley of the Dark Lords to commune with the ghosts of the ancient Sith. This met with about as much success as your average episode of Ghost Hunters. Zilch. He returned, once again a failure, but still treated as the potential Sith savior for some reason. He was about to burst into tears when it was suddenly announced that everyone, regardless of how useless they were, was to be promoted to Dark Lord and sent to battle on Ruusan. All Bane had to do was sit back and accept the title... but he messed that up, too, and cussed out his teachers and ran away, stealing the principal's spaceship.
After flouncing out of the Brotherhood of Darkness for the second time (he was permabanned by now), he wound up on Lehon, the tropical planet where Darth Revan had helped to forge his Sith Empire. Revan had left behind a holocron filled with PBS documentaries like How to Create a Sith Empire, Killing People: A DIY Guide, Force Lightning for Dummies, and so on. Bane studied and learned and gained levels and hit points. Finally, Bane got to the final lesson in the holocron: Kill Everybody You've Ever Met. So Bane decided to kill the entire Brotherhood of Darkness. And the Jedi, too. And also that squirrel. *zap! squeak!*
He was just about to leave and begin his galaxy-wide killing spree when he bumped into Githany and decided to postpone the killing for another 20 minutes. Unfortunately, Githany had coated her lips with five separate poisons, so Bane spent the next few hours crawling to the nearest town for a cure. It all worked out in the end. So, in addition to his other countless failures, he added "failure to die when administered several lethal doses of poison."
Bane: "Of course. Githany refused our in favor of yours. We had to know what he told you about us."
— A meeting between Bane and Kaan
To everyone's surprise (especially Githany), Bane showed up on Ruusan. He told the Sith of the Brotherhood that he had a "cunning plan" to kill all the Jedi. Lord Kaan, leader of the Brotherhood, asked him what the cunning plan entailed, and Bane replied, "With a thought bomb." Kaan raised an eyebrow and asked, "Do you mean to say, Lord Bane, you want us to think the Jedi to death?" Bane said, "Uh... yeah. Basically." There was a very long pause. Then Kaan said, "Great, I love it! Tell us how."
Bane gave Kaan some ancient Sith manuscripts that contained instructions on how to harness the dark side into a thought bomb. Then he left, claiming he had something vital and important to do... somewhere outside the blast radius. "What blast radius?" asked Kaan. "Nothing, nothing," said Bane. "Just learn how to do the thought bomb thing. Everything'll be A-OK."
So Kaan gathered the entire Brotherhood to recite the thought bomb chants, and exactly a hundred Jedi ran in to stop them, and both groups were destroyed, including Githany. Bane crossed out all the names on his list of everyone he needed to kill. Then, to his surprise, he bumped into Little Orphan Annie, who'd just committed an act of murder. This provided Bane the perfect opportunity to take an apprentice... someone who was not only utterly evil, but also cute as a button, so no one would ever suspect. The girl revealed her name as Zannah.
Not to be confused with the Rule of Stew. From now on, Bane declared, there will only be two Sith. Two shall be the number of the Sith, and the number of the Sithing shall be two. Three shalt there not be, nor shalt there be one, unless thou proceedest on to two. Four and five are right out. Besides, there were too many Darths around. Darth This, Darth That, Darth Badda-Bing, Darth Badda-Bang. There were dozens of the blighters. You couldn't throw a dead cat without hitting a dark lord of the Sith, and throwing dead cats is precisely the sort of thing the Sith enjoyed.
The two then proceeded to train in the ways of the dark side and lay the groundwork for the future millennium of the Sith. Just the creepy guy with eye makeup and his underage female apprentice. Make of it what you will. Plus, Bane was kinda on the rebound, after engineering his girlfriend's death and all.
"O great Lord of the Sith Freedon Nadd, share with me your se—gah, gah, get 'em off me, get 'em off me! Damn you, Freedon Nadd!"
―Darth Bane, learning more secrets of the ancient Sith
Bane and Zannah traveled to Dxun to learn from the spirit of Freedon Nadd, but as soon as Bane activated Nadd's holocron, he found himself covered with orbalisk barnacles that fused to his skin and refused to budge. He pulled and he tugged, but they wouldn't come off. His lightsaber wouldn't burn them off. The tattoo removal company failed and was forced to give his money back. Bane was really pissed off now because not only was his body covered with the things, he also ended up with two on his tongue, and several on his penis.
Give him credit, Bane went on studying the dark side and stirring up trouble for years despite being covered in parasites... sort of like Christina Aguilera.
Eventually, he got in a fight with Jedi and got some Force lightning reflected back in his face, which killed the orbalisks... and also poisoned him again. Bane apparently had a very low saving throw versus poison. But he got better and founded the real Sith dynasty, without any of the wishy-washy Brotherhood people. He also went on to write several bestselling evil holocrons, including How I Harnessed My Hate of Constantly Getting Screwed Over by the Universe to Become a Dark Lord and Poison: WTF?.
Oh, and at some point he must've knocked up some Duros chick, because Cad Bane was his great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-grandson.