Stormbreaker

"A king's weapon. Meant to be the greatest in Asgard. In theory, it could even channel the Bifrost." "Does it have a name?" "Stormbreaker."

- Eitri and Thor

Stormbreaker is an enchanted axe used by Thor. It was forged from Uru in the heart of a dying star on Nidavellir. Eitri claimed the weapon was the strongest weapon in Asgard history.

History
"With the iris closed, I can't heat the metal." "How long will it take to heat it?" "A few minutes, maybe more. Why?" "I'm gonna hold it open." "That's suicide." "So is facing Thanos without that axe."

- Eitri and Thor

During the conflict with Thanos, Thor ventured to Nidavellir with Rocket Raccoon and adolescent Groot to forge a weapon that can kill the Mad Titan. Upon arriving, the typically lively forges of Nidavellir were cold and barren, littered with broken parts. The initially scared Dwarf King Eitri found himself reunited with his old friend, and explained that the Dwarves had been rendered all but extinct and his hands had been fused off with metal as a result of Thanos's request of the forgery of the Infinity Gauntlet. With Thor and Rocket Raccoon's help, Eitri reignited the dying star and re-lit the forge, melting down ingots of Uru and pouring it into a mold. He then shattered the mold, revealing the axe's blade and weight, but misplaced the handle for the weapon. Seeing that Thor was close to death after taking the burnt of the dying star's power, Groot decided they could not wait for Eitri to find or create a metal handle; he grasped the red hot pieces of metal with one of his arms and then cut the arm off to serve as the handle, truly finishing Stormbreaker. The weapon then revived the smoking husk of Thor, who used its power of the Bifrost to go forth to Wakanda and assist his fellow Avengers. Stormbreaker was used in the Battle of Wakanda to great effect, augmenting Thor's own lightning massively. It was, notably, the only weapon that caused any noticeable harm to Thanos during the entire conflict: Thor impaled Thanos using it just after Thanos completed the Infinity Gauntlet; however, he intentionally aimed for the chest instead so he could gloat to Thanos, resulting in Thanos only being badly injured rather than instant killed. This proved to be a fatal mistake as the blow could not stop Thanos from activating the Infinity Gauntlet, thus wiping out half the universe before escaping, with the wound closing soon after upon Thanos activating the Gauntlet.

Capabilities
"Wait, what kind of weapon are we talking about here?" "The Thanos killing kind." "Don't you think that we should all have a weapon like that?" "No. You simply lack the strength to wield them. Your bodies will crumble as your minds collapse to the madness."

- Star-Lord and Thor

Stormbreaker is able to focus and enhance Thor's powers. Due to it being composed of Uru, the weapon is nigh-indestructible and able to withstand energy blasts created by the Infinity Stones. It is also able to channel dimensional energy and act as a Bifrost Bridge, which enabled Thor to teleport himself, Rocket Raccoon and Groot to Wakanda.

Like Mjølnir before it, Stormbreaker will return to Thor's hand when summoned, and strikes his opponents with massive force. It glows with an otherworldly blue flame that crackles with electricity, and seems to have the ability to home in on a target if Thor wills it. In addition, the weapon allows the user to fly seamlessly while using it, unlike Mjølnir, which required the user to spin it at great speed to lift them off the ground.

"You should have gone for the head."

- Thanos to Thor

Due to its increased size and mass, having both the blade of an axe and the head of a hammer in its construction, as well as sporting an elongated handle enabling the weapon to be wielded two-handed, it is a notably more potent melee weapon than Mjølnir that came before it. Eitri even went as far as to say Stormbreaker is the strongest weaponry in Asgard's history. As a weapon designed to kill Thanos, it is one of the few able to harm and injure him, with Thor managing to inflict considerable pain and injury when he pushed the axe deeper into the Titan’s chest. Thanos himself, mocking Thor for failing to prioritize a killing blow, suggested that a blow to the head would have killed him. It is so powerful that when charged to Thor's full power, it was capable of overtaking the completed Infinity Gauntlet, quickly slicing through the beam unleashed upon Thanos.

Trivia

 * In the comics, the Stormbreaker was created by Odin as a weapon for Beta Ray Bill, after he returned Mjølnir to Thor.
 * The MCU version of Stormbreaker shares similarities with the Mjølnir of the Ultimate Universe such as the tip of the hammer.

Behind the Scenes

 * When asked how can Stormbreaker overpower the power of the completed Infinity Gauntlet, writers Christopher Markus and Stephen McFeely suggest that Eitri is responsible because of his "Dwarven magic": He made both items and made it so that Stormbreaker is able to defeat the Infinity Gauntlet.