Darth Malak used to be a Jedi until he and his Master found a star map leading to the Composer. In its damaged state, the Composer corrupted the two Jedi and they fell to the Dark Side of the Force. Malak and Revan started building an army but Revan was soon discovered and supposedly killed by the Jedi. (In reality he was betrayed by Malak, but no-one knows this...)
Malak constructed a huge Star Forge around the Composer, and used it to harvest stars to build a fleet, rather like this failure. However, Malak was attacked by the Republic Fleet at the conclusion of the Rise of Trask Ulgo, and was slain by his former master, Revan (who had been reconditioned by the Jedi). His beloved flagship the Leviathan was destroyed, along with his pathetic Star Forge.
Most accounts claim that when Revan went to kill Malak, he started dancing to a tune called 'Dance, Dance, Malak' and then inextricably turned into a dancing Twi'lek. Revan engaged him in a dancing competition, and when it looked like Twi'lek-Malak would win, Revan simply cut him down.
Malak has no hair, apparently he was born this way and despite this fact, he still uses his personal medical droid to shave. Strange. Also, all of Malak's apprentices look exactly the same. Every Dark Jedi without a face mask on bears a striking resemblence to Bandy...