The shapeshifting Skrull Khn'nr was bonded with the DNA of Mar-Vell to lock his body into Mar-Vell's form and given technological replicas of the Kree Nega-Bands. However, his mental conditioning was botched, causing Khn'nr's personality to be erased leaving the Mar-Vell persona dominant. Though part of the Skrull invasion of Earth, this Captain Marvel decides to fight against the invading Skrulls.
The real Mar-Vell died of cancer so to explain his comeback from death, a false memory was implanted in the Skrull Mar-Vell to make him believe he had encountered a wrinkle in space-time sometime during the past, supposedly caused by Tony Stark, Hank Pym, and Reed Richards' construction of a prison within the Negative Zone, which Captain Marvel touched and was transported through time to the Negative Zone in the present day. Initially unsure of how to approach the situation, the Earth's heroes revealed Mar-Vell's history to him but also offered him a place in this new future as a warden for their prison, at least until, as the Sentry implicated, Mar-Vell would return to his past.
He is later called in to help the pro-registration heroes who revived him, in the final battle of Civil War; however upon seeing the chaos they are causing, he departs, to France, where he spends all of his time observing a painting of Alexander the Great in the Louvre and contemplates Alexander's similarities to himself. This painting was meant to be psychological trigger, however, due to an error in the methods of Skrull scientists, the personality of Mar-Vell as well as all of Mar-Vell's memories prior to his Skrull captivity were retained in their entirety and that most of Khn'nr's personality had accidentally been erased as a byproduct of the botched mental conditioning. Consequently, the Mar-Vell persona remained defiantly dominant.
When he learned the truth and became fully aware of his true identity he decided to embrace Mar-Vell's memories and rebel against the Skrulls and protect Earth from their invasion. However, at the same time that Skrull infiltrators strike S.H.I.E.L.D., S.W.O.R.D., the Baxter Building and a number of Stark Enterprises facilities, Captain Marvel launches an attack on Thunderbolt Mountain, deciding that he can destroy some of the thing he sees as problems in the world, like the Government sanctioned supervillains Thunderbolts, during the initial strike of the Skrulls' invasion. He defeated the Thunderbolts, leaving Norman Osborn to convince Khn'nr that only he can decide who he really is, which leads Khn'nr to attack the Skrull armada.
Barely alive after his effort, he manages to crash back on Earth, crossing Noh-Varr's path, who briefly mistakes him for the original Mar-Vell. Before passing of his wounds, Khn'nr spurs Noh-Varr into continuing Mar-Vell legacy as the protector of Earth, branding with his dying words the Skrulls as liars and traitors.
The Skrull also developed technological replicas of the Kree Nega-Bands, though it is unknown if these work in the same fashion as the Kree version which converted Mar-Vell's psionic energy into strength, a high degree of imperviousness to harm, the ability to project force blasts, survive in space and the power to fly at faster-than-light speeds, the bands do give him the exact same powers. They are also fully compatible with the originals, as seen when Ms. Marvel, armed with the original Nega-Bands is able to banish him in the Negative Zone by clicking her wrist on his.