When did Darth Plagueis die in the Star Wars timeline? First mentioned by Emperor Palpatine in Revenge of the Sith, the legend of Darth Plagueis is used as a key step in the process of turning Anakin Skywalker to the Dark Side. The story tells of a powerful Sith Lord who could use the Force to create life and defy death, but was killed in his sleep by his own apprentice.
This apprentice is later revealed to be Palpatine himself, and with the wrinkly old Emperor set to return in the upcoming Star Wars 9, Plagueis' abilities of resurrection could prove a vital cog in the conclusion of the Skywalker Saga. Of course, some theorize that Plagueis never actually died at all - that Palpatine underestimated his master's manipulation of the Force and, to paraphrase an old Jedi Master, that striking Plagueis down made him more powerful than could have possibly been imagined. In either case, Palpatine's assassination attempt is a key event in the Star Wars canon. But when exactly did it take place?
Sheev Palpatine is born around 82 BBY (before Episode IV's Battle of Yavin) and The Phantom Menace occurs in 32 BBY. During this period, Plagueis found Palpatine and began training him as a Sith. However, this is where the story deviates as a result of Disney's purchase of Lucasfilm. According to the Star Wars Legends section of the franchise, Palpatine began his training under Darth Plagueis in 67 BBY and then broke Bane's Rule of Two by training Maul as an apprentice while Plagueis was still alive. Palpatine eventually assassinated his master only after being elected as Supreme Chancellor in 32 BBY in order to take a new official apprentice, rather than Maul who was technically a "Sith Assassin" and permitted with Plagueis' blessing.
Unfortunately, Disney's takeover of Lucasfilm rendered large swathes of Star Wars material non-canon, including plenty of stories involving Maul, Palpatine and Plagueis, such as James Luceno's 2012 Darth Plagueis novel, and the date of the assassination is far harder to place as a result. Another work by Luceno, Tarkin, is still considered canon, and reintroduces a few elements of Palpatine's backstory. While it's confirmed that Plagueis aided Palpatine in his early political career, there's no confirmation of when the future Emperor turned on his master under the Mickey Mouse regime but, interestingly, nothing in the official canon contradicts the events of Darth Plagueis and the legends timeline could still hold up.
The date of Plagueis' death in the modern canon largely hinges on how seriously Palpatine took the Rule of Two and whether or not the Emperor played a role in the miraculous conception of Anakin Skywalker. If Plagueis was killed before Maul became a Sith apprentice, this would put his demise around the 50 BBY mark, but if Maul is once again considered a rare permitted breach of the Rule of Two, then the assassination might've occurred much later, when Palpatine was (possibly) involved in creating Anakin 9 years prior to The Phantom Menace, and therefore readying his own official apprentice. This eventuality would move the death of Darth Plagueis by 10 years or so to around 40 BBY.
If Anakin's conception occurs before Palpatine murders his master, then the new canon likely follows the same timeline as Legends, as Sidious wouldn't have had any cause to assassinate Plagueis until his political ascendance was complete. With that said, the Emperor's first mention of Plagueis to Anakin in the Star Wars films implies many years have passed since the Sith Lord was killed, although it's perfectly plausible the villain might've stretched the truth to make his pitch more appealing.