Khn'nr was a Skrull who volunteered for service in the planned invasion of Earth. Using a DNA sample, he was locked into the form of the long dead Mar-Vell. Replicas of the Nega-Bands were carefully manufactured and given to him as well. However, the Skrulls handling his memory fabrication were not so careful. They rushed the process and attempted to infuse him with the last batch of memories, which included the image that would later unlock his Skrull persona, while he was conscious. This caused a psychotic break. Khn'nr broke free, and believing himself to be Mar-Vell, he killed his perceived Skrull enemies to escape into the Negative Zone.
Khn'nr drifted in the Negative Zone for an unknown amount of time. His mind repressed the memory of escaping the Skrull facility, and in his confused state, he was drawn to a bright light. This light turned out to be the product of Reed Richards' and Tony Stark's experiments to create a Negative Zone portal for the 42 prison. He was transported through and wound up at the feet of the Sentry, who he remembered as an old friend. Sentry consulted with Richards and Stark, and they came to the conclusion that Mar-Vell had been transported forward in time.
Khn'nr was apparently created by Paul Jenkins and Tom Raney in the Civil War: the Return oneshot. However, it is not entirely clear if this Captain Marvel being a Skrull impostor was always the plan and who then was actually the creator of Khn'nr. Jenkins and Raney were the creators on the actual story Khn'nr first appeared in as seemingly Mar-Vell, but Mark Millar was the architect of Civil War, which included Mar-Vell's alleged return. Confusing the issue even more, Brian Michael Bendis was the architect of Secret Invasion and the idea of this Captain Marvel being a Skrull impostor could have easily been his.
Finally, it is possible that it was originally supposed to be the actual Mar-Vell who returned during Civil War and this was changed later to be a Skrull impostor. In that case, Brian Reed and Lee Weeks could be considered the creators of Khn'nr since they set up and revealed this idea in their Captain Marvel limited series.
Major Story Arcs
For further details: Civil War
Khn'nr was told that he was fated to return to the past and die of cancer, which he was already plagued with. He was also asked to help in this time until he returned. Becoming the warden of the new 42 prison, his presence was kept secret from Earth. In truth, that was the purpose of making him warden. Stark wanted him kept someplace safe until his mental state could be determined.
Civil War put a hitch in that plan. Anti-registration forces led a breakout of the prison, and Khn'nr opted to stand by no longer. He put on his Nega-Bands and joined the climatic battle in New York City. He was exposed in the process, but the chaos of the fight still kept him hidden from many. Spider-Man saw him and immediately assumed he was a clone created by Stark. During the fight, he warned a civilian to stay clear and that the world shouldn't be like this. That woman would take those words and later call herself Mother Starr, founder of the Church of Hala.
I Am Here
For further details: the Initiative
Khn'nr took off after the war's final battle and lost himself in the world. He found himself repeatedly drawn to the Lourve and specifically Charles Lebrun's "Alexander in Babylon." Little did he realize that the painting was meant to be the image keyed to unlock his Skrull persona. Because of the faulty memory upload, it only left him with the nagging sense that it should have some meaning.
He was found and approached by SHIELD agent Heather Sante, who had been tasked to monitor him. Stark was concerned that should anything happen to him then the past would be compromised. His talk with her helped convince him that he should stop hiding. He reappeared in New York, making a very public appearance in aiding the Avengers. Now, the whole world believed Captain Marvel was back from the dead.
He was given an apartment in Avengers Tower, where he was secluded. Most of his time was spent obsessing over the Charles Lebrun painting as he was convinced it had something to tell him. He couldn't shake the feeling that there was something important he couldn't remember and that it was just sitting there on the edge of his mind.
During this time, he was contacted by Hulkling, the son that Mar-Vell never knew. Khn'nr did not know how to react when Teddy told him the truth. Eventually, he came to terms with it and contacted the boy again. He confirmed what the Super-Skrull said of Teddy's origin. Mar-Vell and Princess Anelle were together for a short time. They hugged, and Khn'nr told Teddy that he would see him again if time permitted him to before sending him back to his own time. Unfortunately, he would not see Teddy again.
Meanwhile, Khn'nr was also finding himself persistently under attack by strange enemies. Cyclone and Cobalt Man, who were both supposed to be dead, attacked him at different times. Then he and Iron Man were attacked by a unit of Kree soldiers who should not have been able to leave Kree space due to the Phalanx conquest. These were Skrull agents, trying to snap Khn'nr out of it and retrieve him. Their attempts failed. He interrogated the Skrull posing as Cobalt Man to find out why the Skrulls were after him and was told the truth about himself. He was Khn'nr of the Skrull Empire and not Mar-Vell of the Kree. Khn'nr reacted badly. He didn't believe it and killed the Skrull.
In desperation, he came to Mother Starr and the Church of Hala. He needed to do something he could feel still had meaning and thought she could help him do such things. He wound up helping with aid in the Sudan, only to be attacked by a militia. The militia didn't stand a chance against him. All it did was enrage him. He declared that he would see peace on Earth and destroy anything standing in the way of that. These declarations concerned Stark and Carol Danvers, and they decided they needed to get him under control. Using Mar-Vell's real Nega-Bands, Carol clashed one against Khn'nr's and launched him into the Negative Zone. When Tony and Carol couldn't locate him in the Negative Zone, they assumed he had been sent back to the correct time.
What actually happened was that Khn'nr found the portal that originally sent him into the Negative Zone. It was the portal that led to the secret Skrull facility where he was turned into Captain Marvel. As he stepped into the facility, the memories of what happened there unlocked in his mind. He realized that he really was a Skrull. Agent Sante was there as well, having already found the place. She confronted him with the truth she'd found. He admitted it but told her his every memory was that of Mar-Vell. No longer was he Khn'nr of the Skrulls. He believed himself to be Mar-Vell and the Kree and wanted to live as Mar-Vell would.
For further details: Secret Invasion
Khn'nr worked with Agent Sante to find out more about the coming Skrull invasion. Intelligence was recovered from the Skrull facility, but none of it was useful in exposing the specifics of how the Skrulls were planning to take the Earth. He came to realize that nothing he could do would stop the invasion. The Skrulls would attack, the heroes would fight back, and hopefully the Skrulls would eventually be repelled. That was just the way it was going to be. There was no difference for him to make.
So he turned himself in to the Skrulls and asked to help with the invasion. He wanted to be the one to eliminate the Thunderbolts, so that while he could do nothing about the invasion, he could at least take care of that nest of corruption. In that small way, he hoped to leave Earth a better place. He took off on his assignment, attacking Thunderbolts Mountain and defeating the whole team in moments. When it came time to kill them, he found that he couldn't just kill in cold blood. Norman Osborn picked up on this and spoke to him. Osborn played at his confusion and made him realize what he was doing was the wrong thing to do.
Khn'nr spared the Thunderbolts and took off. He reinforced his belief that he was who he chose to be, and he chose to be Captain Marvel, protector of Earth. Nothing was going to shake him from that now. He assaulted the Skrull fleet head on, destroying as much as he could. Eventually, it became too much for him. He fell back to Earth in flames and crashed in the vicinity of Noh-Varr.
He recognized Noh-Varr as a Kree warrior and spent his dying breaths telling Noh-Varr that the Skrulls could not be allowed to win. Khn'nr died in Noh-Varr's arms, reverting to his Skrull form.
Powers and Abilities
As a Skrull, Khn'nr can shape-shift and be undetected as most Skrulls can. As Captain Marvel, he appears to share all the powers of the original Mar-Vell.