In the Earth X trilogy, Captain Mar-Vell gained prominence in the Realm of the Dead due to the fact he knew he was dead at all. In the afterlife, most people believed they were still alive and everyone who was actually alive was dead instead. Mar-Vell's cosmic awareness allowed him to see the truth of his reality. He passed this knowledge on to a select few he could convince they were dead. He was assisted by the banished spirit of Doctor Strange who was similarly aware due to his body still being alive on Earth. They created the Realm Marvel, the astral forms of heroes like Carol Danvers, Bob Grayson, Richard Rider and Peter Quill who acted as scouts and recruiters. Mar-Vell also greatly increased his personal power in the Realm of the Dead by merging with the Enigma Force bestowed upon him by Arcturus Rann. The power typically used to create Captain Universe created the "Mar-Vell Universe" instead.
Mar-Vell saw a way to change the very nature of reality, and sought to return to Earth. He engineered events so that he would be reborn as a child of the artificial humans, Him and Her. In this manner, Mar-Vell became active on Earth and in the afterlife simultaneously. After three years to grow, the child Mar-Vell teamed with Captain America to seek out items of great power for his quest, including Cloak's cloak, the Eye of Agamotto, Books of the Vishanti and Darkhold, the Cosmic Cube and more. Cap's death in Mar-Vell's service brought many heroes to take up the flag in his name, aiding Mar-Vell.
Mar-Vell had a deeply personal encounter with the Supreme Intelligence, found in hiding on the moon. Mar-Vell could not find Una or any of his fellow Kree in the Realm of the Dead, and learned the collective consciousness of the Supreme Intelligence had stolen their souls for countless generations in creating its gestalt mind. Only Mar-Vell had escaped this bond when he died due to his psychic link to Rick Jones. Mar-Vell took up the hammer of Thor and shattered the Supreme Intelligence's tube, effectively committing genocide as he killed every Kree who ever lived. Still, the Kree now existed in the Realm of the Dead, an endless army of a warrior race recruited by the Marvel Universe in the fight against Thanos and Death.
The final battle with Death herself revealed she was in league with Mephisto, the true Adversary of all that lived. With the Power Cosmic and the last piece of the cosmic consciousness added to his collection on Earth, the Marvel Universe was empowered to defeat Death. He revealed the truth about Death's machinations to her pawn, Thanos, and the Titan took up the Ultimate Nullifier at Mar-Vell's request and eradicated Death, giving rise to a new afterlife guided by the principles Mar-Vell would put forth.
The Marvel Universe built this new heaven with the power of his accumulated items, forging it in the anti-matter heart of the Negative Zone. Through the power of the Cosmic Cube, each spirit would craft their own personal Paradise, a wish-fulfillment exercise that would give them everything they ever desired. Mar-Vell empowered Captain America and others as his Avenging Host to guard heaven and guide spirits from the realm of the dead to Paradise once they were ready to accept their deaths.
But doubt crept into Paradise as Mar-Vell became increasingly distant. The Avenging Host learned that Paradise was seemingly annexing space in the Negative Zone, destroying inhabited worlds in that universe. Questions were also raised about whether "Heaven" should be defined by selfish individual wish-fulfillment. Also troubling was the fact that the child Mar-Vell on Earth was now cut off from his second self. Even Mar-Vell did not know whether Mar-Vell was still on the side of good and humanity.
Reed Richards and the Avenging Host ultimately confronted Mar-Vell, as the Kree army attempted to conquer Paradise and lay claim to all that is. Mar-Vell revealed that struggle was a factor of the old universe, and had no place in the universe he was creating. The Negative Zone was not being destroyed by Paradise, but harmlessly absorbed into its borders behind a Wall. This Wall would keep out those like the Celestials and the Elders who intended to manipulate the people of the universe for their own ends and, as the Wall spread through the Negative Zone portal in the Baxter Building to Earth and beyond, allowing humanity and the other races to make their own choices going forward. The Marvel Universe then passed on the majority of his power to Reed Richards, making him the new Eternity of this universe as it formed. Mar-Vell took what was left of his power to travel onwards, ascending towards the core reality from which all other realities first splintered.
At the far end of the Fault, in the reality dubbed the "Cancerverse", Mar-Vell chose to defy death. Extra-dimensional entities known as the Many-Angled Ones reached out to Mar-Vell on his deathbed and promised him life and power. Mar-Vell agreed and, along with the assembled heroes at his bedside, welcomed the Many-Angled Ones into their world and their souls. Now possessed by undying Lovecraftian horrors, Lord Mar-Vell and his "Revengers" sought out Thanos, the Avatar of Death, and sacrificed him in a ritual known as the Necropsy. In doing so, they killed Death herself, making it impossible to die in their universe.
Like an unchecked tumor, life in the Cancerverse grew beyond the space provided by its universe. Space itself became fit to burst with the malignant festering life it contained. When a T-Bomb explosion erupted in frayed space in the Marvelverse, the Cancerverse pushed through. A corridor known as the Fault formed that connected the two universes. In order to colonize this new ground, Lord Mar-Vell sent through his Quasar and aligned with the local Avatar of Life, Adam Magus, to engineer a second Necropsy in this universe.
Mistress Death had resurrected the Thanos of the local universe as her unkillable Avatar of Death, and the Guardians of the Galaxy connected Thanos's return with the rise of the Cancerverse. Adam Magus destroyed entire worlds full of his devotees from the Universe Church of Truth to open the passage at the Fault and allow Lord Mar-Vell and his undying fleet to invade. Thanos and the Guardians slipped by Mar-Vell, however, and entered the Cancerverse to strike at the root of the problem. When Lord Mar-Vell found Thanos in the Cancerverse, the Mad Titan claimed he wished to die and return to Mistress Death's realm, and willingly submitted to the Necropsy. Mar-Vell gleefully killed Thanos again, but in his haste he neglected the specifics of the ritual. By killing Thanos in the Cancerverse instead of his native universe, Mar-Vell did not kill the Marvelverse's Death -- he invited death back into the Cancerverse. The touch of Mistress Death killed even Mar-Vell, and through him annihilated the power of the Many-Angled Ones supporting him. The Cancerverse collapsed in on itself, closing the Fault and ending the threat to the neighboring universe.
In the Ultimate Universe, Pluskommander Geheneris HalaSon Mahr Vell came to Earth in a manner very similar to his counterpart in the core reality. The Kree tracked the progress of the genocidal swarm Gah Lak Tus to Earth. Mahr Vell was sent ahead of the swarm to infiltrate the Earthlings and monitor their development and reaction to the swarm threat, gathering data for the Kree. The Kree were non-humanoid in this reality, and so Mahr Vell underwent genetic surgery to pass for human, contracting for S.H.I.E.L.D. as a civilian scientist named Philip Lawson. He, too, came to work for the space program in Cape Canaveral under security chief Captain Carol Danvers.
The Asis space flight program "Lawson" worked on and coached could potentially have saved some of Earth's population from Gah Lak Tus. However, his commander Yahn Rgg had been driven mad by the knowledge of Gah Lak Tus's true nature. Instead of monitoring the Earthlings, he intended to sabotage their efforts and sent a Killform to destroy Asis rather than allow the human species to survive. Mahr Vell had come to care for the Earth people, and he revealed himself to S.H.I.E.L.D., destroying the Killform and defecting to the side of Earth. With the help of the Ultimates and the Fantastic Four, the newly dubbed "Captain Marvel" destroyed Yahn Rgg and the Kree under his command, while stealing all the available data on Gah Lak Tus for Earth's defense. In the weeks that followed, Captain Marvel and Earth staged the first successful planetary defense against Gah Lak Tus, driving off the swarm.
Captain Marvel remained a hero of Earth after that, working at the command of S.H.I.E.L.D. under his liaison officer, Carol Danvers. He defended Project: PEGASUS against the insane machinations of Reed Richards and was present at the coming of Thanos. When space-time was broken, however, the Galactus of Earth-616 arrived in Earth-1610 and merged with the Gah Lak Tus swarm, becoming an immensely powerful new entity. Hala itself was threatened by this new Galactus, and Mahr Vell revealed the Kree themselves were secretly responsible for creating the original Gah Lak Tus biomechanoids. He returned to Hala to defend his homeworld alongside Rick Jones and Silver Surfer. Mahr Vell died in battle with the swarm, but passed on his bio-technology armor to Rick Jones, giving the Earthman what he needed to help defend both their worlds.
In What If? (1st series) #17, Ronan was not so forgiving when Yon-Rogg triggered the Kree Sentry to attack Cape Canaveral while Mar-Vell was infiltrating it. The colonel was stripped of his command and fled to Earth, challenging Mar-Vell directly. Captain Mar-Vell defeated Yon-Rogg since he neglected to use a breathing potion, and therefore could not intake Earth's atmosphere. Ronan arrived and judged Yon-Rogg, executing him and naming Mar-Vell as new commander of the Earth mission. As Colonel Mar-Vell, the Kreeman was no longer observing Earthlings in the field, but coordinating the mission from a distance. The Watcher predicted that Mar-Vell would therefore not develop any sympathies for the people of Earth, and might one day threaten them instead.
In another reality from What If? (1st series) #12, Captain Marvel bonded with a version of Rick Jones who had become the Hulk in that fateful gamma bomb instead of Bruce Banner. The beats of their relationship was similar to the main reality, with Rick preferring to release Mar-Vell from the Negative Zone instead of the risk of unleashing the Hulk. Finally, shortly after Captain Marvel was exposed to Compound 13, the Rick-Hulk battled Annihilus in the Negative Zone. Bruce Banner's Gammatron device fired through the Baxter Building's N-Zone portal and separated Rick and Hulk into two beings, leaving Hulk to wander the Negative Zone unmolested while Rick returned to Earth, separate from Mar-Vell. Captain Marvel spoke of a curative backlash he received from the Gammatron through their link, implying that in this reality he would never develop cancer from exposure to the nerve gas.
What If? (1st series) #20 retold the events of the Kree-Skrull War, where Ronan impulsively murdered the captive Rick Jones before his human psychic potential could be unleashed. When Captain Marvel tried to contact Rick through the Omni-Wave and found nothing, he flew into a rage. Mar-Vell defeated the Super-Skrull and joined Princess Anelle and the Avengers in leading a revolution against Dorrek VII. Meanwhile, the Supreme Intelligence was forced to physically merge with the brain dead Rick Jones to access his psychic potential, creating a hybrid entity resembling a Supreme Intelligence / human fetal crossbreed. The Skrull civil war led to Dorrek's death, while the new Supreme Intelligence killed Ronan and called for an armistice. As Anelle became queen of the Skrulls, the Supreme Intelligence approached Captain Marvel and named him ruler of the Kree in its stead.
In What If? (2nd series) #14, Captain Marvel detected his cancer using his cosmic awareness months earlier, giving his friends and allies more time to address his condition. Doctor Strange provided access to the Planes of Rashnu, a timeless realm where Mar-Vell could remove the Nega Bands to prevent them from interfering with his treatment without allowing the cancer's unchecked progression to overtake him. Reed Richards and Donald Blake came up with a treatment that cured him from his disease. Flush with victory, Captain Marvel decided to take it upon himself to end the Kree-Skrull War forever. He confronted warring ships and demanded an audience with representatives from each race for peace talks.
However, it was soon discovered that Mar-Vell's cure had unexpected and tragic consequences. He was now a walking plague, transmitting a contagious form of his cancer to anyone who crossed his path. The Fantastic Four were the first to suffer on Earth, while his Kree and Skrull representatives brought the sickness back to their home planets as well. It wasn't until after the Thing died of cancer that Reed Richards realized Rick Jones seemed immune to the infection. Extrapolating a cure based on Rick's previous connection to Captain Marvel, Richards devised a means to stop the disease if it was caught quickly enough. There was no treatment for Captain Marvel this time, though. Unable to cope with the thought of being a plague carrier, Mar-Vell allowed Doctor Strange to exile him to the Planes of Rashnu for all time. Elysius had also contracted Mar-Vell's disease and, fearing the cure might not sustain her, she chose to go into exile to the timeless realm with her love as well.