Darth Revan
Revan was a Jedi Knight and general who turned to the dark side in the Mandalorian Wars. After the death of Mandalore the Ultimate, Revan claimed the title of "Dark Lord of the Sith". He then founded a powerful new Sith Empire, and declared war on the Republic and the Jedi themselves. After defeating almost all of the Republic territories, Revan was captured by the Jedi due to the betrayal of his apprentice Darth Malak. He almost died (a small spark of the Force kept him alive) and was brought to the Council to have his mind replaced by a new identity that was loyal to the Republic. Even after discovering his former identity, he rejected the dark side. Revan ended the war he had begun by killing Malak. After the Jedi Civil War, Revan was given back his rank as Jedi Knight at age 26.