Force Drain: Darth Nihilus requires
vast amounts of life energy to feed his power. He is able to drain an entire
planet and leave it a barren waste land.
Destructive Power: When Darth Nihilus came to feed on the planet
Katarr (home to the Force Sensitive Miralukas) it was said by the sole
survivor, Visas Marr, that he "Spoke, and everything died." He was able to
destroy an entire race (as well as a group of Jedi Masters and Knights) and
turn the planet into a barren waste land with a word.
Soul Transfer: Darth Nihilus was able to store his very soul within
his armor (however this was mentioned once. It is unknown if this took place
before or after his death)
Corruption: Being in Darth Nihilus' very presence caused people to be
corrupted and become slaves to Nihilus. However, this took time and didn't
happen instantly. He was also able to control (and possibly even mutate)
Telekinesis: Darth Nihilus was able to not only lift a massive
Starship, but also hold it together for many years during space travel. It
is unknown if the engines worked or if the ship moved through his will
Star Wars: Tales #24, "Unseen, Unheard"
Star Wars: Knights of the Old Republic II - The Sith Lords (first
Darth Bane: Rule of Two (mentioned only)
Star Wars: Legacy #5 (appears in holocron)