Darth Plagueis was a wise Dark Lord of the Sith who possessed a vast knowledge of the dark side of the Force. He was a Muun male, and had orange eyes. He was a meticulous planner, who tried to accomplish the Sith’s goal to replace the Galactic Republic with a Sith Empire. Due to his powers, he also became paranoid, believing that the Force could «strike back».
Obsessed with achieving immortality, Plagueis strived to discover the path towards becoming an eternal being alongside his apprentice Darth Sidious and rule together for thousands of years, and believed the secret to it laid in science. He desire to cheat death represented a subversion of the natural order of things and an ultimate act of manipulation of the the universe and the will of the Force. While Plagueis became afraid of losing his pupower, he never expected his apprentice to be the reason he would lose power. Ultimately, Plagueis had trusted and underestimated Sidious, who betrayed and murdered him when he least expected it. Though Plagueis did not live to achieve his goals, Sidious succeeded to survive his death, but given that his transfer was imperfect, he asked himself if maybe Plagueis got the last laugh after all at his failure.
Darth Plagueis was first mentioned on-screen in the 2005 film Star Wars: Episode III Revenge of the Sith, the third and final installment of the Star Wars prequel trilogy. Although Palpatine was never confirmed to be Plagueis’ apprentice in the movie itself, a link to the official encyclopedia on StarWars.com did refer to Sidious as having been «trained by Darth Plagueis.
The character was created by George Lucas as early as the first draft of Revenge of the Sith—dated April 2003—and possibly earlier. His story was massively expanded upon in the 2012 Star Wars Legends novel Darth Plagueis, written by James Luceno. It notably established Plagueis to be a Muun just as Lucas proposed.
Although Legends works show that Plagueis had power over life and death, Lucas himself stated Palpatine’s tale about Plagueis’ abilities was a lie. The Biography gallery of Darth Sidious’ Databank entry states that Plagueis was killed by Sidious after he took Darth Maul as his apprentice. However, the 2019 reference book Ultimate Star Wars, New Edition lists Plagueis’ murder at Sidious’ hands as taking place before Sidious takes Maul under his wing.