Darth Plagueis is the overarching antagonist of the Star Wars franchise and the titular villain protagonist of the Star Wars: Darth Plagueis novel in the Legends timeline.
Born under the name Hego Damask II (in Legends) he is a Muun Dark Lord of the Sith, heir to the lineage of Darth Bane, apprentice to Darth Tenebrous and a master of midi-chlorian manipulation who lived during the century leading up to the Invasion of Naboo. Obsessed with eternal life, Plagueis experimented with ways to cheat death and create new life from the midi-chlorians.
His great contribution to the history of the galaxy was training his apprentice Darth Sidious in the ways of the Sith and the dark side of the Force, whom he incited to take control of the galaxy and bring about a new age of the Sith. Sidious, convinced that his master had outlived his usefulness, eventually betrayed him by killing him in his sleep.
Years later, Sidious (in his civilian guise of Supreme Chancellor Palpatine) recounted Plagueis' story as The Tragedy of Darth Plagueis the Wise, an old "Sith legend", to sway Anakin Skywalker to the dark side.