The Marvel Universe is full of immensely powerful weapons, and many of those weapons have shown up in Marvel Studios' films. Nowhere is this more prominent than in the Thor films. Of course, there's the hammer Mjolnir and its counterpart Stormbreaker; both hammers allow Chris Hemsworth's God of Thunder to harness the storms that are his namesake. The Casket of Eternal Winters can blanket its victims in endless frost, while the Eternal Flame empowered the burning god Surtur and his Twilight Sword. Thor: Love and Thunder will continue that tradition with All-Black the Necrosword, which Gorr the God Butcher (Christian Bale) uses in his mission to slaughter every deity who ever lived.
In the comics, the Necrosword is a weapon that gives its user powers close to a god. Whoever wields it becomes encased in a cloak of living shadows, and has their strength and speed boosted to inhuman levels. While wielding the blade, Gorr has brought down dragons and even carved a moon in half! He can also craft the shadows surrounding his body into monstrous beings known as "Black Bersekers". True to their name, the Berserkers are little more than brutes driven by an unquenchable fury - and with Gorr's ability to create endless hordes of Berserkers, they're a formidable force whether you're a god or not. And the Necrosword can even contain its host's consciousness, as Gorr seemingly perished in battle against Thor but reappeared to torment him during the King Thor miniseries by Jason Aaron and Esad Ribic.
The history of the Necrosword is just as bloody as that of the being wielding it. It came into Gorr's possession when he encountered a pair of gods that were locked in battle. Furious at the deities for not answering his prayers while his wife and children suffered and died, Gorr took the Necrosword and used it to kill them both. He continued to use its powers to kill gods across time and space, as well as crafting shadowy images of his long-dead wife and son Agar. During his battle with Thor, Gorr was confronted by his victims, who revealed that by amassing power and spreading his legend across the universe, he had become the very thing he hated: a god. Stricken with horror, Gorr was eventually defeated by Thor, who wrested the Necrosword from him and used it to slay the God Butcher.
Gorr is not the only being to wield the Necrosword. In King Thor, the blade would ultimately land in the hands of Thor's brother Loki - transforming him into a malevolent being obsessed with killing the universe. It would also possess the Devourer of Worlds Galactus and Ego the Living Planet before resurrecting Gorr. Gorr revealed that he attempted to recreate his efforts with the Godbomb, a device he had created in his youth to wipe all deities from existence - only this time, he'd rigged the entire universe to blow. An elderly Thor would give his life in order to stop Gorr from killing the universe, leaving his daughters to watch over Midgard.
The weapon used by Gorr in the movies is not nearly this powerful. While Gorr can still use it to craft Black Berserkers, they differ in size and shape - they're even different creatures, which adds variety to the level of threats that Gorr can send against Thor and his allies. The Necrosword in the movie also allows Gorr to move through shadows - making him extremely hard to hit. But even though the God Butcher is more than a match for the deities he preys upon, his blade comes with a horrific curse as it slowly kills whoever wields it. This provides a dark parallel to Jane Foster (Natalie Portman) and her journey in the movie. Jane gains the power of Thor when she picks up his reforged hammer Mjolnir; however, Mjonir's magic accelerates her cancer and ends up shortening her life.
The Necrosword has a connection to another Marvel character, as it isn't just a sword - it's a symbiote. As in, the alien creature that bonded with both Spider-Man and Venom. The full story came to life during Venom #4 by Donny Cates and Ryan Stegman: All-Black was forged by the god of the symbiotes Knull in order to wage war against the Celestials, as Knull viewed the massive deities as a threat to his reign of eternal darkness. With All-Black, Knull crafted a legion of symbiote soldiers and battled the Celestials' forces – with one of his vessels and a Celestial soldier falling to Gorr's planet. Knull would eventually regain control of the Necrosword and attempt an invasion of Earth in the King in Black miniseries by Cates & Stegman, though Eddie Brock would ultimately slay the dark deity and take both his blade and his title. It seems highly unlikely that this connection will be touched upon; in the film's final act, Jane destroys the Necrosword by trapping its fragments within Mjolnir and obliterating them with lightning.