Darth Plagueis
I know you have probably heard of Darth Plagueis because of when Palpatine was telling Anakin about him in Revenge of the Sith, but you probably don't know really anything about him. Plagueis was Darth Sidious's master. He was so powerful that he could use the the Force to create life and keep people from dying.
Darth Bane
Darth Bane was a legendary Sith Lord who lived hundreds of years before the Clone Wars. As the only surviving Sith, Bane realized that Sith infighting had weakened them to the point that the Jedi were able to destroy them. Because of this, Bane created a new wave of Sith and wrote the Rule of Two.
Starkiller
Starkiller was originally going to be Luke's last name, but George Lucas changed it to Luke Skywalker because it didn't sound as violent or things like that. The name was reused as Darth Vader's apprentice.
Darth Talon
Darth Talon was actually a girl Sith. She was also a Twi'lek, the same species as Jedi Aayla Secura and a few others. Talon was the apprentice of Darth Krayt, another Sith who is on this list. She was covered in black Sith tattoos. Each tattoo had been earned in combat and given to her by Krayt.
Darth Krayt
As it says above, Darth Krayt was the master of Darth Talon. He was a Jedi during the final days of the Republic. His master is unknown