Nebulos is a being that resided in a foreign dimension and ruled over the Planets Perilous. At some point in time he obtained the Staff of Polar Power, which enabled him to absorb evil mystic energies from other beings.
Strange Tales I #161 - #163
Nebulos's existence is first revealed when he encounters Doctor Strange on his Planets Perilous, who arrives there with Victoria Bentley after Baron Mordo banishes them to Nebulos's dimension. Nebulos decides to trick Strange into becoming his mindless servant and orders him to grab the Staff of Polar Power, which gives Nebulos control over Strange's very being. Once Strange is corrupted, Nebulos sends him back to Earth to confront Baron Mordo while wielding the Staff. This allows Strange to absorb all Mordo's power into the staff before he banishes Mordo into another dimension. However, after he obtains Mordo's evil energy he is brought back to Planets Perilous by Nebulos's power. Nebulos then calls the Staff of Polar Power to himself, pleased to having taken Baron Mordo's evil energy.
Unfortunately for Nebulos, the power absorbed from Mordo caught the attention of the Living Tribunal, who arrives on Planets Perilous and tells Nebulos that he will destroy the Staff of Polar Power. Nebulos threatens to unleash all the evil energy in his Staff, which prompts the Living Tribunal to battle him. The two entities struggle to wrest the Staff out of the other's grasp, with Strange attempting to help the Living Tribunal by attacking Nebulos with spells - although both Nebulos and Living Tribunal are annoyed by Strange's interference. The Living Tribunal realizes that all power beamed against Nebulos would be absorbed by his Staff, and as Nebulos tries to run away he uses a Flames of the Faltine spell to begin destroying all of Planets Perilous. Nebulos still refuses to part with his Staff even though his home is being destroyed, but Doctor Strange whisks away the Staff just before Nebulos is buried in debris from the crumbling planet.
It is unknown if Nebulos was actually destroyed or merely incapacitated. However, he has never been seen since.
Nebulos could obtain power from an evil mystic being by absorbing the evil magical energy with his Staff of Polar Power. The Staff also demonstrated the power to take over the minds of others, as well as enable one to travel between realms freely. Nebulos was able to struggle against the Living Tribunal due to the power contained in the staff, but was still no match for Living Tribunal's might. His own personal power outside of the Staff appears to be rather low, a crumbling planet was enough to seemingly kill him.