Darth Plagueis, also known as Darth Plagueis the Wise, born Simeon Gome, was a Jedi Master in the lineage of Darth Bane. He devoted his life to midi-chlorian manipulation, and taught everything he knew to Ethan Richeter, his first apprentice. His journey to become stronger in the force was only possible through the Dark Side of the Force. He became a sith and took upon a new apprentice, Darth Sidious, who he also taught everything too.
Plagueis at one time was the strongest person in the galaxy, both in the force and lightsaber combat. He eventually lost his two arms and was unable to use his lightsaber form. Sidious was taught an easier version of the form. When Sidious believed he was strong enough, he killed Plagueis, killing him in his sleep.