In 1947, the Prime Skrull (last of his kind) fled to Earth to escape extinction. He and his pursuer crashed on Earth, and wound up in the custody of human scientists such as Dr. Caitlin Maddox, who each spent decades studying them.
The Peacekeepers attempted to take custody of the Prime Skrull, but were thwarted by the Underground Legion, and only made off with the deviant Skrull who had pursued him.
Captured alongside the Underground, Dr. Maddox was taken into custody at the base, and placed in a laboratory with the Prime Skrull's body. To her surprise, she discovered he was still alive.
Dr. Maddox managed to escape with the Prime Skrull, reviving him with the Skrull adrenaline samples they had made. Although he was barely conscious, his ability to transform matter proved invaluable when he changed the plasma bars holding the rest of the Underground Legion into solid matter. The Underground brought the Prime Skrull to safety, escaping the base.
The Underground brought the Prime Skrull back to William Amos' mansion, where Dr. Maddox cared for him as though he were a child, nursing him back to health.
After learning that Tantalus had used a stargate to return home, the Underground determined to follow him, and made use of the Prime Skrull, whom they needed to navigate a vessel for them.
On their way after Tantalus, the Underground encountered the Kree Starforce, who joined forces with them, informing them that they would find Tantalus at Armechadon.
Upon their arrival at Armechadon, the Underground and Starforce joined forces with the Underground of Armechadon against the armies of Tantalus.
In final battle with Tantalus, the Prime Skrull confronted the Skrull pilot, and used his power to make the Skrull lose control over his body, becoming grotesquely elongated.