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darth plagueis is an ancient sith lord, and one of the dark lords who followed darth bane's rule of two. plagueis furthered the teachings of the sith, unlocking the deepest, darkest secrets of the force.
darth plagueis became a powerful sith lord during the thousand years leading up to the battle of naboo, and took on a sith apprentice he named darth sidious. sidious' early life remains shrouded in mystery, though he learned much from plagueis, and grew even more powerful than his master - perhaps the most powerful sith lord in history.
plagueis realized sidious could well implement the final stages of the revenge of the sith, and began to teach his apprentice his most important secrets. in his study of the dark arts, darth plagueis had somehow found a way to cheat death; to manipulate midi-chlorians to sustain and even create life through the force. in a twist of fate, it was the secret of eternal life that led to plagueis' doom. as soon as sidious learned his master's secrets, he killed plagueis and became the new dark lord of the sith.
(burda sanki biraz spoiler
darth sidious used the teachings of darth plagueis to extend his own life, to ensure the culmination of his plans. sidious later told the tragedy of darth plagueis the wise to anakin skywalker, luring him to the dark side with the notion of eternal life. skywalker, plagued by disturbing visions of the death of his wife, was soon corrupted. unfortunately, anakin was forced to rely on the secrets of the sith to sustain his own life after suffering near-fatal injuries at the hands of obi-wan kenobi on mustafar.
source: revenge of the sith
-- behind the scenes --
darth plagueis was revealed by spy reporters months before the release of revenge of the sith, though the episode iii lead-in novel, labyrinth of evil, was the first publication to officially name sidious' master. the name is derived from the word 'plague', continuing the tradition of the darths adopting titles associated with evil or the dark side.