"No, I mean against our common enemy!"
- -Lord Kaan and those who would become the Brotherhood of Darkness.
Forming the Brotherhood
Kaan was a member of one of the various Sith Empires that used to go around terrorizing the Galaxy. He was discouraged by the fact that Sith Lords, being the jerks that they were, spent most of their time fighting each other for power, rather than using their powers together to kick the Republic's ass. He decided to promote a theory that all Sith Lords should be equally powerful and feared. Many of the combatant Sith Lords eventually saw his point and organized as the Brotherhood of Darkness. With everyone cooperating, the Sith grew very powerful and were able to take control of large portions of Republic space.
When Lord Hoth, consolidated his Jedi forces in an attempt to stop losing, Lord Kaan saw an opportunity to wipe out all the Jedi at once, so he consolidated all his Sith Lords as well. What followed was a drawn-out series of battles on the planet Ruusan. Niether side could completely eliminate the other, and both sides had a stream of unprepared recruits to throw into the line of fire. Eventually, Darth Bane was kind enough to provide Kaan with a Thought Bomb, some sort of magic spell or something that would destroy all the Jedi. Bane conveniently left out the part about it killing all the Sith as well. Kaan "detonated" the thought bomb and killed the entire Brotherhood, and some Jedi, including Lord Hoth, while Bane giggled from behind a protective wall.