Darth Malak Profile
Once Darth Revan's pupil, until he "killed" his master by ordening the destruction of his spaceship, then he proclaimed himself the new Sith Lord. Once both were Jedi until Revan discovered the location of something called the "Star Forge", which was said to be a source of unlimited energy and power. They both then were concidered to have fallen to the dark side of the force, and Malak indeed did so. He became obsessed by the power of the Star Forge and decided he would be the ruler of the entire Galaxy. Unfortunately, he misunderstood Revan's true intentions.
Malak's envy and hate towards his master led him to kill him, like it always happens with every Sith Lord and their apprentices. After that, Malak focused on destroying the Republic and on finding Bastilla Shan, probably the only Jedi who could stop him due to her Battle Meditation power. In the end he is able to capture her and even to turn her into his apprentice, but he didn't count with one thing: Revan was alive. While Revan had lost his memory (but later regains it) he was a danger for Malak, and while he was surprised, he was also happy to see his old master alive since he always felt that his assumption of the title of Dark Lord of the Sith was too easy and wanted to kill Revan with his own hands.
Both do their final battle inside the Star Forge, where Malak discovered a way to suck the force out of other Jedi into himself, making him more powerful. In the end, depending on which side Revan chooses, he either ends up killing Malak and regaining his position as Sith Lord and of the Star Forge, or killing Malak and feeling sorry for leading him to that path.
Malak, like most of the Sith, seeks only to rule the entire galaxy and gain as much power as he can. One notable aspect of him is the absense of his jaw, replaced by a metal cover which makes his voice sound more "metallic", while it isn't really explained, it was done by Revan when Malak didn't agreed with his true plans.
Game of appearance: Star Wars: Knights of The Old Republic
