Darth Plagueis

""Darth Plagueis was a Dark Lord of the Sith, so powerful and so wise, he could use the Force to influence the midi-chlorians to create life. He had such a knowledge of the dark side, he could even keep the ones he cared about from dying.""

- ~Chancellor Palpatine

Darth Plagueis is part of the Muun species, and is one of the most powerful and wise Sith lords ever since the near-destruction of the Sith, and the development of the plan of the Sith existing only as one master and one apprentice.

Plagueis took Darth Sidious as his apprentice ever since Sidious was a child. When Plagueis discovered how to create life by influencing the midi-chlorians, young Sidious discovered that his master was planning to replace him. In order to prevent that from happening, he killed Plagueis in his sleep.

In Revenge of the Sith
Darth Sidious tells Anakin about Plagueis's ability to influence the midi-chlorians as part of his plan to bring him over to the dark side. In the movie, Anakin believes that his wife Padme will die, but Sidious uses this to manipulate Anakin into believing he can save her and this eventually brings Anakin to the dark side.

Trivia
Unlike other characters who lived before the movies took place, Darth Plagueis has not made any significant in-person appearances in the "Expanded Universe". This is most likely to maintain his enigmatic image as an unseen character from Revenge of the Sith.