WARNING: The following contains spoilers for The Immortal Hulk #34, by Al Ewing, Butch Guice, Tom Palmer, Paul Mounts and VC's Cory Petit, on sale now.
The Marvel Universe has usually painted other Hulks or Hulk-like figures as the Green Goliath's greatest enemies. Inside his own mind, he's got Joe Fixit to deal with, not to mention Green Scar and the Devil Hulk. Externally, General Ross as the Red Hulk or even the Abomination often top the lists when it comes to Hulk's most dangerous figures.
However, The Immortal Hulk #34 confirms Bruce Banner's deadliest opponent isn't even a Hulk: it's Samuel Sterns/The Leader. And while in the past, the Leader has been a threat due to his genius, now it's because of the way he's mixing his scientific knowledge with the supernatural realm.
While Immortal Hulk has featured a lot of science with the Minotaur, Roxxon and Shadow Base hunting the Jade Giant, gamma has been retconned to be connected to the One Below All. This new, magical take sees gamma connecting people to the Green Door, which is the passageway the One Below All's uses to manipulate gamma-irradiated beings -- including Banner. But what's scary about the Leader is he's found his own way of harnessing the door and has given himself willingly to the One Below All. As he recalls all the times he died, the Leader reconciles data and keeps levelling up with each resurrection. He's doing it at will, entering the door and leaving when he wants to.
He's more in control than ever, and with all the gamma experiments he's done on himself, as well as subjects like Soul Man or Ross' Red Hulk, science and the supernatural are part of him in a big way. He's going through all of the data and the remnants of the A.I. Gammon, upgrading his mind and body. More so, he's learning more about Banner and his Hulks, and the fact he's also worked with Mephisto suggests he could have more secrets hidden away.
Mephisto and the Leader's relationship didn't end well in Thunderbolts, but the latter admits he got a lot of info for his occult studies. This could be anything from spells to a secret deal, so by tinkering and experimenting with all these heroes and villains, he keeps evolving into the ultimate foe. Everything from mind control and necromancy are a part of him, giving him that cutting edge that enables him to be the One Below All's official avatar.
This unlimited power has allowed Leader to co-opt Brian Banner, Bruce's dad who was trapped in hell, so he knows the weaknesses of the Hulk no one else does in full. He's always been a mental threat but now, Sterns is imbued with everything he needs to make a physical impact too. The Leader has long searched for it, and now with this lethal formula is in his hands, and a dark god behind him, it won't just be the Hulk, but the entire planet in trouble. And due to the power of the Green Door, once Stern cranks it up, it's unlikely Hulk will be able to muster a solution.