Although the moniker of "Captain Marvel" is most commonly associated with Carol Danvers, it originally belonged to a Kree spy-turned-superhero named Mar-Vell, who first debuted in Marvel Super-Heroes #12 (by Stan Lee, Gene Colan, Frank Giacoia, Stan Goldberg, and Artie Simek). After Mar-Vell died of cancer in the 1982 graphic novel The Death of Captain Marvel (by Jim Starlin, Steve Oliff, and Jim Novak), his title was traded between several successors before it was eventually claimed by Carol, one of whom was Mar-Vell's son Genis-Vell.
In many ways, Genis-Vell appeared to be the ideal heir to the Captain Marvel name, inheriting even more powerful versions of many of Mar-Vell's iconic powers thanks to his unique heritage. Unfortunately, Genis-Vell's incredible powers were also a great burden, and his inability to control them would ultimately transform the son of Mar-Vell into one of the Marvel Universe's greatest threats.
While saving the Eternals' homeworld of Titan from a genocidal supercomputer known as I.S.S.A.C, Mar-Vell met and fell in love with Elysius, an Eternal who served as I.S.S.A.C's chief lieutenant. Reciprocating his feelings, Elysius betrayed her master and helped Mar-Vell, Drax the Destroyer and Rick Jones save Titan. Sadly, Mar-Vell and Elysius would only get to spend a few blissful weeks together before Mar-Vell's untimely death. Grief-stricken, Elysius used the advanced technology of Titan and samples of her and Mar-Vell's DNA to conceive a child, whom she named Genis-Vell.
Knowing that her late lover had many powerful enemies, Elysuius artificially-aged Genis-Vel into adulthood before hiding him on the planet Caclulex. During a chance run-in with the Silver Surfer in Silver Surfer Annual #6 (by Ron Marz, Joe Phillips, Bob Almond, Ericka Moran, and Ken Bruznak), Genis-Vell discovered his true heritage and set out to continue his father's heroic legacy.
As the son of a Kree super-soldier and a god-like Eternal, Genis-Vell possessed several impressive abilities, many of which were more powerful versions of the ones his parents possessed. These included super-human strength, flight, and the ability to absorb and manipulate cosmic radiation, which Genis-Vell could use to blast beams of energy and heal other living beings. In time, Genis-Vell's radiation-based abilities would improve even further, allowing him to create powerful forcefields, conjure illusions, and achieve a form of immortality.
However, Genis-Vell's most distinctive ability was his Cosmic Awareness, an ability that his father had received from the cosmic entity Eon. This ability granted Genis-Vell omniscience, allowing him to be aware of everything and everyone within the universe.
However, Genis-Vell's great power came at an even greater cost. As Genis-Vell's powers grew, his ability to control them did the exact opposite, taking a deep toll on Genis-Vell's mental state. In particular, his Cosmic Awareness' ever-expanding range forced Genis-Vell to witness the limitless possible futures born from his every action or inaction, often showing him the worst possible outcome of every decision he made.
Driven mad by the constant stimulation, Genis-Vell developed a twisted god-complex, using his increasingly-powerful abilities to bend the universe and the people within it to his will. Eventually, Genis-Vell would destroy all of reality by killing Eternity, the physical embodiment of the Marvel Universe, although it would be quickly restored soon after.
Following this, Genis-Vell would go on a series of increasingly bizarre adventures, battling his sister Phyla-Vell and killing his time-traveling son Ely-Vell, which drove Genis-Vell even deeper into madness. Eventually, Genis-Vell would recover some of his lost sanity with the help of Rick Jones, an old friend of his father's. Hoping to atone for his misdeeds, Genis-Vell joined a new team of superheroes known as the Thunderbolts. Unfortunately, the Thunderbolts' leader, Citizen V, would eventually be unmasked as the supervillain Helmut Zemo, who killed Genis-Vell and scattered his dismembered remains across the Darkhold Dimension, ending the bloodline of Mar-Vell once and for all.
Genis-Vell's downfall is a tragic end to the lineage of one of Marvel's greatest heroes. Although Genis-Vell initially wanted to be a hero like his father, he lacked Mar-Vell's discipline and inner strength, which allowed his ultimate power to corrupt him. However, Genis-Vell's death did ultimately allow Carol Danvers, who was also a close friend of Mar-Vell, to claim the name of Captain Marvel and redeem Mar-Vell's legacy.