Seeking an heir, Kang the Conqueror fathered several sons, each called Marcus, throughout time with different women (mostly divergent iterations of his love Princess Ravonna). Kang killed the majority of his sons when each proved an unworthy successor, although Marcus Kang XXIII showed promise, he too was ultimately murdered by Kang. Kang continued to attempt to find an heir, a quest that presumably continued into his transformation into Immortus. Immortus and Revelation conceived Marcus Immortus, and the child was born in the Limbo Dimension where he was forced to grow up by himself unable to ever leave for fear of disrupting the timestream.
Marcus Immortus was created by David Michelinie and George Perez and first appeared in The Avengers #200.
Marcus Immortus' only major role in a comic book was in his first appearance in The Avengers #200. The issue depicted a largely nonsensical plot which saw Ms. Marvel suddenly pregnant with Marcus Immortus' child who, when born, turned out to be Marcus Immortus himself who instantly grew to adulthood and left to rule Limbo with Ms. Marvel. The story was almost universally derided by readers due to it's incomprehensible plot and it's treatment of Ms. Marvel who was victim to an off-panel rape, a subject entirely side-stepped by the issue. Marcus Immortus apparently won the undying affections of Ms. Marvel and the two left to be together forever.
Due in part to the controversial story, Marvel tried to distance themselves from it and Ms. Marvel was de-powered and didn't appear in books for years to avoid the issue. Marcus Immortus was revealed to have died off-panel and his treatment of Ms. Marvel was confirmed as forced upon her and against her will in Avengers Annual #10. To this day, the exact nature of Ms. Marvel giving birth to the man who impregnated her has not being brought up or explained and writers and fans alike prefer to ignore the story rather than try to make sense of it or justify the treatment of Ms. Marvel's character in it.
With that said, Marcus Immortus briefly returned in Avengers: The Terminatrix Objective, where his true mother was revealed as being Revelation (like Immortus to Kang; the final iteration of Princess Ravonna). Although he appeared in all 4 parts of the storyline, he did not feature heavily and nor was his history with Ms. Marvel brought up.
Ms. Marvel later met an earlier son of Immortus, Marcus Kang XXIII, during Kang Dynasty where she was forced to face her past with Marcus Immortus. Marcus Kang similarly fell deeply in love with her, but despite flashbacks depicting her and Marcus Immortus, no new information or understanding of the original story was presented.