Darth Plagueis/Legends

Darth Plagueis is a fictional character in the Star Wars universe, who is mentioned in passing during Star Wars Episode III: Revenge of the Sith.

He was apparently the Sith Lord to whom Darth Sidious was apprenticed. However Sidious betrayed Plagueis and killed him, thus becoming the Sith master himself. Plagueis is sometimes refered to as "Darth Plagueis the Wise", as apparently he found a way to manipulate midichlorians to prevent death.

He is also mentioned in the novel Labyrinth of Evil.