Step into the (Green) Light; Green Door

 The Green Door is a metaphysical barrier that connects the Below-Place, the bottom layer of the Multiverse even below the deepest layer of Hell, to the real world. 

Green Door Tag

After being killed, Gamma Mutates can come back to life by passing through the Green Door, effectively making them immortal.

Origins

Weird Science: Gamma Mutate

A genetic mutation triggered by exposure to gamma radiation physically transforms the subject.  This often results in a monstrous appearance and drastic changes to their coloration. 

History

After death, the souls of gamma mutates cross into the Below-Place through the Green Door. They can then return to life when another Green Door manifests for them, though they possess no memory of this following their resurrection. The first known opening of a Green Door in Earth-616 took place in 9500 BCE, when a young Jordanian man named Tammuz was killed by a radioactive meteor called the Good Green Eye, which allowed Tammuz to then come back to life as a gamma mutate. In the modern day, a Green Door was opened in the Los Diablos Missile Base in New Mexico due to the detonation of the Gamma Bomb built by Bruce Banner, which caused his death and his subsequent resurrection and transformed him into the Hulk.

Through a Green Door, the One Below All can influence gamma mutates, albeit it needs a host personality to act through. It eventually found one in the form of the soul of Bruce Banner's father, Brian, who was plunged into the Below-Place after death. Using Brian as a conduit, the One Below All began to possess and manipulate gamma mutates for its own goal, which was to destroy everything and become the sole presence in the Multiverse. After possessing the Absorbing Man, the One Below All used him to soak up all of the ambient gamma radiation left in the Los Diablos Missile Base and opened a Green Door into the Below-Place. The Hulk and Bruce were split apart, and Brian attempted to turn Bruce into the new vessel for the One Below All, but the plan failed, and the Hulk managed to merge back with Bruce and close the Green Door, returning to Earth.

When he put into motion a plan to help the One Below All bring forth a future in which he succeeded in his plan, the Leader used his occult knowledge to open a Green Door into the Below-Place, where he assimilated Brian Banner and became the One Below All's new host. The Leader began using Green Doors to control Rick Jones, Del Frye and partly the Hulk himself. The Leader also demonstrated the ability to alter a mutate's Green Door, changing it into a Red Door to prevent their resurrection, as he did with Leonard Samson's Green Door when it manifested while he was in the Below-Place.

While Bruce Banner shut the Green Doors to prevent the One-Below-All from escaping again, removing the Gamma Mutates' immortality in the process, Doctor Strange used Betty Ross as a conduit to open a Green Door in the Fifth Cosmos to defeat Moridun with a font of magical power surpassing his own.

At some point, due to his nature as a Gamma-irritated being, the Spider-Man villain Digger also managed to return to life after passing through the Green Door from the afterlife and reconstituted himself. He came under the employ of the would-be New York crime-boss The Rose.