Darth Revan

Darth Revan was a Jedi who lead the Old Republic to victory over an uprising by the Mandalorians approximately 4000 years BBY. After defeating the Mandalorians, Revan and his apprentice, who would become Darth Malak, sought and found a mysterious power source that they used to found a new Sith Empire and attack the Republic.

During a Jedi raid on Darth Revan's flagship, Darth Malak betrayed his master, ordering his own ship to open fire on the flagship. Revan was seriously injured in the attack and captured by the Jedi.

To say more would be spoilerific.