Darth Plagueis was Darth Sidious's Master, Until he was killed during his sleep by Sidious himself. Darth Plageuis had the ability to bring people back to life and was the Leader of the Galactic Empire until killed by Sidious, which Sidious then took the spot as Leader. He was introduced when Sith lord Darth Sidious was talking to Anakin Skywalker, When he was trying to turn Anakin to the Dark Side of the force.