Warning: Contains spoilers for Hulk vs Thor: Banner of War Alpha #1
After Thor had to destroy Mjolnir to get rid of the soul of the evil Mangog infesting it, the All-Father of Asgard had his hammer reforged, only to find out that the spirit of the deceased Odin now resides inside the weapon. While Thor may not be happy about his father's persisting influence in his life, this new upgrade makes Mjolnir and the God of Thunder more powerful than they have ever been.
Mjolnir was originally forged by the dwarven smiths of Nidavellir under the orders of Odin when the King of Asgard needed a way to trap the God Tempest, a powerful and unruly primordial being he had bested in battle. While proving to be a mighty weapon, the God Tempest's lingering spirit made Mjolnir a whimsical tool, so the One-Eyed God decided to leave it for his son Thor to inherit when he proved to be worthy. Recently, the spirit of the deceased Mangog, one of Asgard's most relentless enemies, found a way to bond with the God Tempest inside Mjolnir, creating a vengeful and almost unstoppable creature that called herself the God of Hammers and went so far as to kill Odin. To defeat her, Thor had to destroy Mjolnir. The spirit of his father, finding Valhalla to be razed and destroyed by an unknown enemy, has now come back and taken residence inside the weapon.
When Thor learns from his trusted ravens the location of the fugitive Hulk, sought by the Avengers for causing a massacre in El Paso, he immediately goes to intercept the Green Goliath. Itching for a fight to forget about the many unpleasant events that followed his ascension to the throne of Asgard, Thor wastes no time in attacking Hulk, now in one of the most powerful forms ever thanks to Bruce Banner's insane "improvements." The battle between these two powerhouses starts in Hulk vs Thor: Banner of War Alpha #1, by Donny Cates, Martin Coccolo, and Matt Wilson. The fight is, as is to be expected, brutal, but Thor seems to have an edge thanks to Mjolnir's new status as the receptacle for Odin's soul.
The hammer can now move on its own, meaning it can hit enemies even while Thor is incapacitated as it happens in this case, with Hulk caught completely flat-footed by Mjolnir hitting him in the back at Mach speed. Also, while Thor is understandably annoyed by Odin's voice in his head constantly telling him what to do, it is also true that his father's spirit is millions of years old, and spent the majority of that time on a battlefield gaining invaluable experience that Thor now has access to. Finally, in this issue Mjolnir displays a new, tricky power: after striking Hulk, Odin's spirit "invades" the Jade Giant's body, and is now about to do battle with Bruce Banner inside his "mindscape."
Thor has lost Mjolnir many times in recent years: first when he became unworthy during Original Sin, then when the hammer was destroyed by Jane Foster to kill the Mangog, and finally to get rid of the God of Hammers. Just when it seemed that Thor's struggles with his weapon were over, he has to deal with the constant presence of Odin inside the hammer. The God of Thunder is very unhappy about this situation, but the truth is that his fight against Hulk proves that Odin's soul makes Mjolnir, and Thor, more powerful than they have ever been.