Deep in Earth’s future, the time-traveling conqueror Kang came to find his excursions into time left him unfulfilled, and he decided to give up worldly conquests and meditate on the nature of time until the end of his days. Thus, he journeyed to Limbo, a dimension that exists outside the timestream itself and built a castle, lording over the wanders of Limbo, the Space Phantoms.
At some point after this, Immortus was contacted by the trinity of beings known as the Time-Keepers to oversee the 80 millenia of their existence and keep humanity (and especially the hero team known as the Avengers) from expanding into space-- a prospect of which would have disastrous prospects in nearly 50% of all the multiverse. Immortus acted as "custodian" of this timeline, and acted many times to influence events that keep humans from exploring space, all under the guise of embarking “on a number of campaigns to untangle the skein of timelines."
These early machinations included sending a Space Phantom to break up an early incarnation of the Avengers and helping the early version of the Masters of Evil. During this last excursion, Immortus witnessed the bravery of Captain America, which forever would allow him a grudging respect for the Avengers. Thus, instead of destroying the Avengers, Immortus would subversively shepherd them to his own ends, such as manipulating events so that Thor would relinquish his hammer's time-travelling capabilities.
Immortus also set out so that the Scarlet Witch would not conceive children (who have the potential to stand among the greatest forces of the universe.) Immortus helped to instigate the relationship between the Scarlet Witch and the Vision, thinking that no children would be born of this union, and Immortus officiated the ceremony that married the two. When this tactic failed, Immortus investigated the origin of the children, and manipulated events to reveal that the children were in fact pieces of the demon Mephisto's soul. Immortus also took the opportunity to influence the dismantling of the Vision's robotic body and mind. Immortus tried to stop the Scarlet Witch once and for all banishing her from reality, but was instead seemingly banished himself. Immortus was saved by the Time Keepers and re-instated as their custodian.
Immortus ultimately blundered, however, when the Avengers chose to involve themselves in the Kree-Shi'ar War. Immortus had instilled within their team member Iron Man a temporary sense of xenophobia, hoping to drive the Avengers back to Earth. Instead, Iron Man helped other Avengers to execute the Kree leadership embodied by the Supreme Intelligence. This act spawned many possible futures, all of which ultimately showed the Avengers driven into becoming fascist "protectors" of space. Thus, Immortus brought about the very timelines he was charged with preventing. To keep the Avengers distracted, Immortus influenced Iron Man again, this time allowing Iron Man to turn villainous and murder various Avengers associates. Immortus also personally engaged the Avengers under the pretense of being Kang, alongside a version of Mantis and other versions of men and women Avengers had known-- all of which were Space Phantoms in disguise. The tactic proved so distracting that the Avengers never recovered, and when many of their number seemingly sacrificed themselves to end the psychic menace of Onslaught, the Avengers disbanded.
A year later, however, the heroes returned to Earth and soon re-formed the Avengers. At this point, Immortus tried to stop what he believed was the ultimate cause of humanity's destiny by killing Avengers' associate Rick Jones. Instead, Immortus was stopped by Kang, who had just emerged from an identity crisis and would rather destroy Immortus than to be fated to become him. Forced into a war with Kang, Immortus ultimately captured Kang's Forever Crystal. With the Forever Crystal, Immortus could make sweeping changes throughout the timestream just as a gardener would prune unwanted branches. Jones brought various versions of the Avengers throughout time to confront Immortus. The Avengers eventually learned the truth of Immortus' many manipulations and, alongside Kang, the occultist Libra, and the Supreme Intelligence, confronted Immortus and the Time Keepers to convince them of their injustice. To stop Kang, the Time Keepers sought to warp time, to force Kang to bodily merge into Immortus. Kang's will, however, proved too strong, and he ripped himself away. The castoff, Immortus, remained distinct from his previous incarnation, as reality reshaped itself so that both men existed separately at the same time. Immortus, rejuvenated by the experience, vanished into the timestream to contemplate the matter.
At some future point, Immortus will have vanished from Limbo under unrevealed circumstances. During this time, his son Marcus will rule in Limbo.