Revan

Revan was perhaps, the most powerful Jedi in the history of Star Wars lore, and in addition
to his master of the force, and skill with a lightsaber, he was a tactical genius. He easily
rallied the republic and his jedi crusaders against the Mandolorians.

But after the war was over, he took his armada into the unknown regions and found
something unknown. Upon his return, he had a massive fleet of sith at his side, and
declared himself a sith lord. He lead a devestating campaign against the republic while
leaving their infrastructure in tact, for reasons that were unclear. It is speculated that he was
doing this to unite the galaxy against an even greater threat, but before his victory, his
apprentice Malak betrayed him.

The jedi wiped his memory and turned him against Malak and his Sith Empire, he
succeeded and was redeemed in the eyes of the jedi, but yet again recalled the original
threat and departed for the unknown regions of the galaxy, never to be heard from again.