WARNING: The following contains spoilers for Immortal Hulk #37 by Al Ewing, Joe Bennett, Ruy José, Belardino Brabo, Paul Mounts, and VC's Cory Petit, on sale now
In all media, the concept of an afterlife is complicated and comic books are no different. With so many differing viewpoints factoring in to create a sense of understanding for readers, it can be hard for writers to create or break the rules of the afterlife while simultaneously having their stories make sense.
In the Marvel Universe, where heroes die and come back so often it's hard to keep track, it can be hard to define the afterlife as something concrete. The afterlife for Gamma mutates, or people empowered by Gamma radiation such as the Hulk or Abomination, has been an ongoing mystery since its introduction but in Immortal Hulk #37 by Al Ewing, Joe Bennett, Ruy José, Belardino Brabo, Paul Mounts, and VC's Cory Petit, the rules of the afterlife for Gamma mutates are made a little bit clearer.
In Immortal Hulk #37, it is revealed that Leader has taken control of the deceased body of Dr. Del Frye by mastering the secrets of the Below-Place, the personal hell that Gamma mutates go to when they die and then are resurrected. Having spent time in this hellscape, Leader has learned how to do what the One Below All can and possess the bodies of recently deceased Gamma mutates by manipulating the Gamma radiation, changing its color from green to red to block souls from returning to life. He even kills Dr. Leonard Samson to prove his power. But his true endgame was always to maneuver the Hulk into a fatal battle, one that was taking place with the Absorbing Man, who had taken in Gamma radiation to battle the green behemoth. As Hulk fought to save his friend, he was struck down, and no sooner had he been felled than the Leader swooped in like a vulture to claim Hulk's body for himself, combining his brain with the Hulk's brawn.
Leader can achieve this feat through the nature of the Below-Place. First introduced in Jim Zub, Mark Waid, Al Ewing, Joe Bennett, and Paco Medina's Avengers #684, the Below-Place is the lowest layer of Hell, being the home and prison of the One Below All, the evil counterpart to the One Above All. The One Below All is the source of Gamma radiation, attempting to manipulate its wielders through power. As a consequence of this connection, even though practically all Gamma mutates had no say in the matter, when they die they are dragged into the Below-Place, a fitting punishment for those who use the Gamma radiation for evil, but a cruel joke for those who try to use it for good. Fortunately for them, the nature of Gamma radiation means they don't stay dead for long. A portal to the living world opens shortly after the mutate in question dies and they return to life with no memory of the horror they traveled to once they slipped the mortal coil.
This portal is referred to as the Green Door and is both the saving grace of the Gamma mutates and the main barricade to the One Below All. Also introduced in Avengers #684, the Green Door acts as the gateway back to life for deceased Gamma mutates, the door also provides the mercy of wiping away the memories of the Below-Place for those who pass through it. Though as of late, forgetting the Below-Place exists has become more of a problem than a blessing. While it may be a door for the mutates, it is the main gate preventing the One Below All from leaving his prison, so even though it was opened by the Gamma radiation bomb that created the Hulk, ironically, it is keeping the progenitor of Gamma radiation in check.