Jedi Vs. Sith, 2001
About 1000 years before Emperor Palpatine overthrew the Republic and restored the Sith Lords to power, the Brotherhood of Sith Lords fought against the Army of Light in the Seventh Battle of Ruusan during the New Sith Wars. Darth Bane was a member of said Brotherhood, but felt that their large numbers would lead to too much infighting and the Sith would end up destroying each other. Bane convinced Lord Kaan (the leader of the Sith) to attempt a Force power known as the "thought bomb" which led to the annihilation of both sides and left Bane the sole surviving Sith.
Bane decided that the Sith needed to operate more secretively and instated the Rule of Two. This tenet stated that there could only be two Sith at a time: a Master and an apprentice, one to hold the power and the other to crave it. The apprentice would attempt to overthrow their master and take on a student of their own. This would keep the Sith constantly powerful, and as we see in the films, it ultimately paid off for Palpatine.
It would be super cool to see Bane show up in a film someday. He's one of my favorite Sith Lords and characters in general, and his story is definitely worth checking out.