Hidden from the galaxy, Palpatine handed his spirit to a failing clone. To find someone to manage his evil power, Palpatine manufactured more clones, including Dathan, who he considered a failure for his Force insensitivity.
Vader's secret son Luke Skywalker destroyed the Death Star, sparking the Rebellion-Empire war. Vader ended the Galactic Civil War by saving his son and throwing his master down a reactor shaft at Endor.
After Count Dooku died, Palpatine turned Anakin Skywalker evil and named him Darth Vader. He issued Order 66, commanding every clone to execute Jedi for treachery, destroying most of the Order instantly.
Palpatine and his new student, Count Dooku, dominated both Clone Wars sides. While the Republic Senate gave Palpatine emergency powers to battle the Separatists, Dooku could pursue the Sith's long-term goals as head of a new nation.
A thousand years after the Sith were thought to have died out, Darth Plagueis and Darth Sidious, his pupil, started putting the last bits of the Grand Plan together.
As of 1032 BBY, the Sith were at the height of their power. However, their greed turned them against each other, and the Jedi were able to beat them.
A very powerful superweapon was built on Malachor at some point during the Age of the Sith. The Jedi struck the Sith temple before it could be used.
A rebellious Jedi was kicked out of the Jedi Order for using the dark side of the Force. Over time, this Jedi got other Jedi to join his cause.