Darth Plagueis was a Dark Lord of the Sith who, at some point, chose Sheev Palpatine, a Human from Naboo, as his apprentice. As master and disciple, Plagueis and Palpatine—who took the Sith moniker Darth Sidious—worked together, trying to unlock the secrets of immortality and to execute their order's long-percolating plan of supplanting the Galactic Republic with a new Sith Empire.
Darth Sion, one of my favorites. "If killing you spares you what lies ahead then kill you I must. It is not possible to walk away from such things unscarred... To keep living while the universe dies around you. She will try to break you, teach you how far you can fall. You are her weakness, as you were mine."