As a result of the Viltrumite Empire’s vicious expansion, the Unopan have been nearing extinction ever since their first encounter with the intergalactic conquerors. In order to stave off this fate, the Unopans established breeding camps — one of which resulted in the birth of Allen the Alien. However, Allen was no mere Unopan spawn. Rather, he was part of a group of fetuses that were subjected to experiments with the goal of increasing their strength exponentially. If successful, the children would be born with the destinies of heroes — if unsuccessful, they would be born horribly disfigured. Allen was the only successful test subject.
The horrifying implications of this practice aside, it did eventually lead to the moment seen in Episode 3, in which Allen’s regenerative evolution should be triggered. In the comics, Allen’s strength, speed, and durability increase so dramatically that he can even defeat Omni-Man with relative ease. After a second near-death experience, Allen becomes so powerful that he’s able to withstand the fires of a star to save Invincible — and though Mark Grayson emerges just short of a charred skeleton, Allen has no visible injuries whatsoever.
Sadly for fans of Seth Rogen’s portrayal in the series, Allen might well be out of commission until Season 3. The character’s “death” seen in Episode 3 took place in Issue #23 of the comics — and he didn’t return until Issue #38. Given the pace the series has maintained thus far, it will be quite a while before we see Allen the Alien on our TV screens.