Millennius was an enemy of Thor and the Silver Surfer, heroes residing in the Marvel Universe - he is a rogue Norse god whose views on humanity have had him banished from his home of Asgard and made him an enemy of the Asgardians.
Thousands of years ago Millennius was an Asgardian god with controversial views regarding humanity. Believing that humanity was cursed with pains and sorrows, Millennius believed that the solution to their problems was to kill them all, granting them eternal peace. His mad philosophies placed him into conflict with Odin and Odin led an army of the most powerful gods of Asgard against him, finally imprisoning him in the Land of the Forsaken. Millennius found only lowly "slugs" within the dimension but eventually put them to work as slaves to construct a castle for himself. When the dimension was invaded by creatures from other realms, Millennius would attack them and gradually gathered enough technology to begin constructing a device which he hoped would release him from his confinement. Every thousand years Millennius's prison would weaken and he would attempt to return to Asgard, but each time he was driven back by Odin. When the Silver Surfer attempted to take Thor back in time so that they could witness who had overthrown Asgard, Millennius detected the Surfer's cosmic power and realized that it could release him from his prison. Since the Surfer was attempting to depart his own plane of existence, Millennius was able to use his machines to bring the Silver Surfer and Thor into his dimension. He then sent the slugs to attack them, and the slugs knocked them both out via suffocation. Millennius placed the Silver Surfer within a machine that would siphon his cosmic power into his escape portal but would also kill the Silver Surfer in the process. Millennius then went to confront Thor and when Thor learned who he was he attacked him, having heard of Millennius as "a legend whose name is used to frighten small Asgardian children." Millennius argued that his goals had been distorted over time but Thor considered him mad and the two fought. The Silver Surfer escaped his bonds but found that he could not seal Millennius' portal because it fed upon his power. The Surfer arrived to aid Thor, destroying Millennius' mace and Millennius was heartened to hear that his escape portal was nearly ready. While the Silver Surfer engaged in combat with Millennius, Thor used his hammer to seal the portal by channeling his own life-force into it. Thor and the Silver Surfer managed to beat Millennius unconscious but were too tired to continue battling him for much longer. To stall Millennius long enough for them to depart his dimension the Silver Surfer encased him within rock and sealed him within the center of the planet he dwelled on and Thor sealed the hole in the planet. They returned to their home dimension and Millennius was left trapped inside the planet's core, determined to escape his prison - whether he ever will is unknown.