The Tragedy of Darth Plagueis the Wise was a "Sith legend" that was relayed to Anakin Skywalker by Palpatine, telling of his master, Darth Plagueis. Plagueis, the legend went, was "so powerful and so wise he could use the Force to influence the midi-chlorians to create life," and even save others from dying. "He became so powerful, the only thing that he was afraid of was losing his power. His apprentice, Sidious"—though Palpatine did not name himself when first recounting the story to Anakin—"killed him in his sleep."
According to Palpatine, what made it a tragedy was that he could not save himself from death, despite being able to save others. When Palpatine told this story to Skywalker, Skywalker countered that it was only a tragedy for Plagueis, and for the apprentice, it was a happy ending. Palpatine later revealed that he was the unnamed apprentice and that Plagueis' ability had been real; this was one of the steps towards turning Skywalker to the dark side of the Force.