On the behalf of this extremely strong and powerful Sith Lord, very little is known of his origin. One thing that is known is the fact that he survived the Mandalorian Wars and the battle on Malchor V; after the wars, with his hatred, he began his way to the Dark Side of the Force.
Nihilus' power grew to the extent that he was consumed by it, becoming a wound in the Force. However, along with his raw power, the Mass Shadow Generator activated at Malachor may have played a part in Nihilus' status as a wound in the Force, but this is not confirmed. Nihilus is known as the Lord of Hunger because of his need to "feed" on living things by draining their Force energy. Nihilus travels across the galaxy in a ship called the Ravager, searching for vast amounts of Force energy for him to drain.
Darth Nihilus was one of Darth Traya's two apprentices along with Darth Sion in the Sith Triumvirate. Eventually, Nihilus and Sion betrayed their master and severed her connection to the Force. A Miraluka planet called Katarr drew Nihilus' attention because of the Miralukas' connection to the Force and the presence of a Jedi enclave on the planet. Nihilus fed on the energy of the entire planet, destroying all life on it's surface, except for a young Miraluka named Visas Marr; it has not been explained how Visas survived the mass genocide Nihilus caused on Katarr.
Visas would afterward train under him for some time. Darth Nihilus' partner, Darth Sion, travelled the galaxy, searching for remaining Jedi to destroy, while Nihilus continued to traverse space searching for Force energy to drain. Upon learning about the Jedi Exile, Nihilus sent his aprrentice, Visas Marr, to locate and attack the Exile, which resulted in Visas abandoning her master and joining with the Exile
Darth Nihilus assisted a rebellion that broke out on the planet Onderon. He joined forces with General Vaklu in an attempt to overthrow the Onderonian queen. With the help of the Mandalorians, the Exile was able to thwart this attempt. Nihilus was then preparing to drain his next target, the planet Telos. Unfortunately for Nihilus, he was tricked by Traya into believing that Telos had an abundance of Force energy present in its population.
This was untrue however, as the planet did not have any Force sensitives present, but despite Telos not having nearly as much Force energy present on its surface as he had expected, Nihilus continued in his attempt to drain it anyway as a result of not having had fed in a long period of time.
He was intercepted by the Jedi Exile, Visas Marr, and Mandalore, who boarded the Ravager in order to stop the Lord of Hunger, and, thus, he never gained the opportunity to drain it. As the Jedi Exile herself was a also a wound in the Force, she was resistant to Nihilus' Force Drain ability, which in turn weakened the Sith Lord further. Eventually, because of Nihilus' excessively weakened state, as he had not fed on Force energy in an extenuating period of time and because the Exile weakened him by resisting his drain, Nihilus was ultimately defeated.
Nihilus' form released Dark Side energy upon his defeat as a result of his potency in the Dark Side. After his defeat, the Exile and the Mandalorians successfully destroyed the Ravager, saving Telos from Darth Nihilus. At some point, Nihilus had created a holocron, which came into the possession of Darth Krayt, millenia later.
Darth Nihilus was an extremely powerful Sith, indulging himself into the power of the Dark Side. However, he became a slave to his own power, resulting in his need to feed off of the Force energy present in living things. He has developed a highly powerful Force Drain, which allows him to drain on a scale as massive as an entire planet. His drain is the means by which he sates his hunger. It has never been specifically defined how often Nihilus needs to feed on Force energy or how much he needs. However, after draining the planet Katarr, he was able to last for an entire year without the need to drain Force energy.
Nihilus also possessed vast telekinetic abilities, which allowed him to construct and hold together his ship, the Ravager. Nihilus was adept in Farseeing, which granted him the ability to sense Force sensitive beings, such as the Exile, from great distances. He also possessed skill in other powers such as Force Scream and Lightning.
Darth Nihilus has also been said to have the power to transfer his soul into his armor, presumably through use of the Force power Transfer Essence. It is unknown if this suggests his survival after his encounter with the Exile. If this occurred prior, then this would mean Darth Nihilus was capable of existing without flesh and blood, and even retain great Lightsaber dueling skills within a suit of armor.