A new Star Wars book has revealed Palpatine wasn't lying when he described Darth Plagueis' abilities to Anakin Skywalker in Star Wars: Episode III - Revenge of the Sith. Palpatine (Ian McDiarmid) has had his sights set on the Star Wars franchise's prophetic Chosen One from the start, realizing the potential threat of a powerful new Force-sensitive being trained in the ways of the Jedi. In the aftermath of the Battle of Naboo—wherein a young Anakin (Jake Lloyd) helps bring down the Trade Federation Army—the new Chancellor swiftly begins to cultivate a relationship with the child prodigy, immediately sensing his power. Sith desire power above all else, and so Darth Sidious decides that Skywalker must fall to the dark side at all costs.
Palpatine's plans build to a head in Star Wars: Episode III - Revenge of the Sith, where he is finally able to take advantage of an older Anakin's (Hayden Christensen) twisted sense of attachment. The Sith Lord cast visions into his mind—images of his wife's death that he knew would push the Jedi Knight to the brink of the dark side. Anakin was unable to ask the Jedi Counsel or even his mentor Obi-Wan Kenobi (Ewan McGregor) for assistance due to the secret nature of his marriage to Padmé, and even if he did they would likely consider his fear of her death to be a forbidden attachment, encouraging him to simply allow the will of the Force to play out. Palpatine struck in a moment of emotional weakness, telling Anakin the story of Darth Plagueis the Wise, a dark lord of the Sith "so powerful and so wise" that he could manipulate midi-chlorians and "even keep the ones he cared about from dying."
Surprisingly, Marc Sumerak's new book Secret of the Sith—purportedly written from Palpatine's own notes—reveals Darth Sidious isn't actually lying about Plagueis' ability. According to the book, Darth Plagueis did indeed learn how to keep someone else from death, although he had not yet worked out how to apply this extraordinary power to himself. Seeing the potential in this, Palpatine killed his master before he could secure the key to immortality, and continued in Plagueis' footsteps, eventually discovering the secret of the dark side's extremely rare essence transfer ability seen in other Star Wars stories, both canonical and Legends.
Given Darth Plagueis' methods later became the basis for Palpatine's own essence transfer, it is reasonable to assume they involve the manipulation of an individual's spirit, perhaps even transplanting them to another body—a decidedly dark side act. Secrets of the Sith further reveals, however, that Plagueis was fascinated by ancient legends of a Dyad in the Force and indeed attempted to form one with his apprentice, Darth Sidious. Plagueis believed the power of a Dyad was greater than life itself, and he was aware of legends suggesting members of a Dyad could bring people back from the brink of death, so he presumably hoped to find another method of immortality from it.
Of course, this all adds another layer of tragedy to the fall of Anakin Skywalker. Palpatine really does possess the secret that would allow him to keep Padmé alive in Star Wars: Episode III, but of course it's not in the would-be emperor's best interest to reveal that information. He ultimately uses Padmé as a necessary sacrifice in order to transform Anakin into Darth Vader. Still, it's fascinating to learn that the greatest villain in Star Wars history wasn't lying when he claimed to possess this power.
