WARNING: The following contains spoilers for The Immortal Hulk #38 by Al Ewing, Joe Bennett, Ruy Jose, Belardino Brabo, Paul Mounts, & VC's Cory Petit, on sale now
There are multiple versions of the Hulk across the Marvel Universe, and most of them reside somewhere in the depths of Bruce Banner's mind. While all of these are among Marvel's most powerful and savage characters, the most dangerous of the Hulk personas is the Devil Hulk, who has been a major force in Immortal Hulk.
And in The Immortal Hulk #38, the Devil Hulk has been fully rereleased within Bruce Banner's mind, and it's ready to go after the Leader, the longtime Hulk villain who's currently in control of another Hulk persona.
Although he didn't debut until Paul Jenkins, Ron Garney and Sal Buscema's Incredible Hulk #13 in 2000, the Devil Hulk was perhaps the first alternate persona that formed within the mind of a young Bruce Banner. Created as a response to his father David's disdain for him, the Devil Hulk represents everything hateful and cruel within Bruce's mind. Fiercely protective of Bruce, the Devil Hulk is a monstrous and murderous figure. Banner spent most of his life containing the Devil Hulk and trying to keep him from taking out his rage on the world. But the Devil Hulk was released upon Bruce's death during the events of Civil War II, and it has since become one of the primary versions of Hulk featured in Immortal Hulk, dishing out cruel justice whenever he gets the chance. It's the Devil Hulk that has motivated Bruce to try and fix humanity by any means necessary, which has set him into conflict with various other incarnations of the Hulk.
But following their battle with Xemnu, the Devil Hulk was seemingly contained and chained down within Bruce's subconscious by the more heroic World-Breaker Hulk, much as the Devil Hulk had been held down in the past. However, it turns out this was really all the machinations of the Leader as part of his efforts to become the dominant force within Bruce Banner's mind. Having usurped power and breached Hulk's mind, the Leader even seems to take control of the World-Breaker persona, becoming one of the strongest versions of the Hulk -- at least within Bruce's mind. But even while confined, the Devil Hulk remains as protective of Banner as ever and is constantly talking about escaping and helping Bruce.
When the Leader threatens to drag Bruce Banner himself through a Green Door to confront Brian Banner and the One-Below-All, the Devil Hulk finally cuts loose. Breaking through the chains that the Leader placed around him, the Devil Hulk lands in front of a terrified Leader, who initially can't believe that the Devil Hulk has escaped his confinement. But the Devil Hulk is unleashed, and ready to destroy as much of the gamma-irradiated being who was once Samuel Sterns as he can.
While this is definitely good in the short run, it could also lead the Devil Hulk to finally lash out at the other personas within Bruce's mind. This could give the Devil Hulk a dominant role in Bruce's mind. This could also be another step towards the dark future where the Devil Hulk has been usurped by the One-Below-All and used to kill every single living thing in the entirety of reality. Regardless of what happens next, the Devil Hulk is fighting for the side of angels for the moment, and he's helping tear down the Leader before he can do any more damage to the world -- or Bruce.