Darth Nihilus came to learn the very greatest of the Sith teachings; however, little is known of Nihilus's true beginnings, some believe him to be a survivor of the Mandalorian Wars, and might have been at one point a prisoner of the Mandalorians. After the war he was found by Darth Traya to be Force-sensitive, and she began training him in the ways of the Sith at the Trayus Academy on Malachor V.

Through a vision Traya shares with the player, it is revealed that after his true powers and potential were realized, Nihilus, along with fellow Sith student Darth Sion, plotted and turned against their dark mistress. They cast Traya down, stripped her of her power, and expelled her from the Sith Order. Their reason for doing this was because Traya insisted on fighting a "shadow war" against the Jedi, as she tells the Jedi Exile. Nihilus and Sion preferred a direct, much less subtle approach. Nihilus chose to destroy the Force-sensitive planet of Katarr (and much of the remaining Jedi Order with it); Sion chose to lead his sect of Assassins in destroying the remaining Jedi.

Darth Nihilus ravaged and consumed whole worlds to feed his insatiable appetite for power. This was conducted from the bridge of his flagship, the Ravager, which appears as a playable location later in the game. Nihilus's powers were so all-consuming that all those who came into contact with the Sith Lord became slaves, mindless servants to his will, as seen in the crew of the Ravager when the player comes across the ship. It is suggested that he drained the Force from these formerly living beings to enslave them.

One such servant of Nihilus was Visas Marr, a Miraluka female from Katarr, a planet consumed and destroyed by Nihilus. The Sith Lord sent Visas on a mission to find and bring the last of the Jedi before him, so that he could consume his power and essence. But this would be the mighty Sith Lord's undoing - the Jedi Exile broke her from Nihilus's bonds after she boarded his ship, the Ebon Hawk, and she subsequently turned against her former master.

Nihilus was informed of hundreds of Jedi on the planet Telos. Setting his fleet out to the planet, Nihilus was surprised when he found out that no "Jedi" were on the planet. His ship was subsequently invaded, and on the bridge of the Ravager, Nihilus attempted to absorb the Exile's Force energies, only to weaken himself, possibly because the Exile was a void in the Force, much like himself. The Exile, together with Mandalore Canderous Ordo and Visas, defeated the Sith Lord on board the bridge of the Ravager. After his death, his body dissolved in an aura of dark side energies. Visas also gives the player the chance to take the Sith Lord's mask after he is defeated.

What happened to Nihilus after his body dissolved is unknown. Whether he is truly gone shall remain a mystery. Interestingly, his true identity is not revealed even when Visas looks at his face after his defeat, leaving questions about whether his previous personality may have been anyone of consequence. Visas, being able to see the world solely through the Force, merely says when she looks upon him that she sees a thousand graveyard planets and an eternity of hate and hunger. It is interesting to note, however, that depending on what path the player character chooses, when Visas looks upon the Sith Lord, she says she sees "Just a man, nothing more." There is much speculation between Star Wars fans about what exactly Nihilus became after he was no longer a man. There is also much speculation about his origins. They will hopefully be answered by a possible sequel.

Like most of the Sith Lords, Darth Nihilus's name has some meaning (nihil is Latin for nothing). He has no loyalties to anyone but himself, and will consume both Sith and Jedi with his hunger, which notes that his name fits his current personality quite well.

*Taken from: http://en.wikipedia.org/wiki/Darth_Nihilus