The following contains major spoilers for Thor #26, available now from Marvel.
In the Marvel Universe, Bruce Banner's Hulk has gone through upgrades time and time again. His splintered psyche has produced unique sentient iterations, such as Joe Fixit and Savage Hulk, not to mention the smarter Professor Hulk of the past. They've been adaptive forms according to the circumstances the Green Goliath found himself in.
However, some upgrades, while they offer more power, are much more twisted, as seen with his Devil Hulk form, and recently, the destroyer known as Titan Hulk. Interestingly, as "Banner of War" continues, the Jade Giant just received his mightiest one yet in Thor #26 (by Donny Cates, Martin Coccolo, Matt Wilson, and VC's Joe Sabino).
It came in the fight in the void known as the Black Hand of God. In his Starship form, Hulk emitted so much gamma energy that Thor got poisoned and turned into a Hulk himself. Iron Man tried to intervene but his Celestial suit was already damaged and could do nothing but watch this maddened Thor smash Starship Hulk. This reaffirms the power of Thor combined with gamma rays results in the ultimate destroyer.
However, Banner worked out a deal with Odin in order to protect the Jade Giant. The All-Father's spirit had already figured out how his gamma energy was corrupting his Asgardian son. Now, he wanted to ensure that curse didn't further harm his son, or Asgard. After all, Thor destroyed the Bifrost bridge when Sif tried to bring him home, so Odin wanted to put an end to this world-breaker. It resulted in the ex-king possessing Mjolnir and flying through towards his son.
Shockingly, when Thor-Hulk tried to catch it, it broke his hand and went to Starship Hulk. It turns out, Odin agreed to share his power with Starship Hulk, converting him to a worthy warrior: the Mighty Hulk. It was quite a glorious site seeing Hulk with his helmet, Asgardian armor and Mjolnir. To add swagger to the situation, he even rocked Thor's hubris, goading him for a brawl.
He told the Odinson if he wanted the hammer, he had to come get it. It resulted in an epic final page where Hulk's hammer and Thor's fist clashed, leaking out massive amounts of mystical energy. This was a true clash of the titans. The moon they were on even blew up.
The story ended there, but with one chapter left, fans are excited to see who is the victor. A Hulk versus Thor fight is a highly in demand fan proposition. The concept has transitioned from cartoons to the Marvel Cinematic Universe in Avengers: Age of Ultron and Ragnarok. But this is one a different level, powering them up like never before and ultimately, giving them a battlefield to cut loose with deadly intent.