Darth Plagueis was a Sith Lord who trained Darth Sidious to use the Dark Side of the Force. According to his apprentice, his power was so great that he could create life and keep other people from dying. Unfortunately, he could not keep himself from dying when his apprentice murdered him in his sleep.
In the Legends canon, he was a Muun banker publically known as Hego Damask, who arranged many of the events that ultimately lead to the rise of the Galactic Empire.
There is a hypothesis among some Warsies that Darth Plagueis foresaw his own death and used his power to "create life" to cause the conception of Anakin Skywalker, who would eventually avenge his murder.