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."  ~Palpatine to Anakin Skywalker

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 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.