Stormbreaker

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

- Eitri and Thor

Stormbreaker is an enchanted axe used by Thor. It was forged from Uru on Nidavellir. Eitri claimed the weapon was meant to become the strongest weapon in Asgard's 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 Groot to forge a weapon capable of killing 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 covered in metal as a result of Thanos's demand for the creation of the Infinity Gauntlet. With Thor and Rocket Raccoon's help, Eitri opened the cage around the 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 brunt of the neutron 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 the Avengers. Stormbreaker was used in the Battle of Wakanda to great effect, augmenting Thor's own lightning massively. Notably, of all the battles, it's the only weapon that severely wounded Thanos by impaling him soon after the Infinity Gauntlet was completed. However, Thor intentionally aimed for Thanos' chest, resulting in Thanos only being badly injured rather than instantly killed. This allowed Thanos to activate the Infinity Gauntlet, and eliminate fifty percent of the universe's life. Moreover, Thanos would also fully recover from said mortal wound by the time he had arrived on another planet to enjoy the sunset. As Thanos teleported away, Stormbreaker fell to the ground, the blade covered in the Mad Titan's blood.

Stormbreaker was later summoned to Thor's hand when he stared down Captain Marvel, testing her resolve. In addition, when the Avengers ambushed Thanos, Thor took great pride in cutting off Thanos' left arm while the Infinity Gauntlet was still attached his hand. When Thanos announced he had destroyed the Infinity Stones, Thor cut off Thanos' head this time and fulfilling what Thanos himself told Thor should have done the first time.

Five years later, during the final battle against Thanos, Thor dual-wielded both Mjølnir and Stormbreaker, using them to great effect in the battle and being able to go head to head with the Mad Titan. However Thanos gained the upper hand and nearly used Stormbreaker to cut open Thor's chest as he had previously done to Thanos.

Thor used Stormbreaker to help repel the combined forces of Outriders, Chutari and the Black Order. In some certain moments, Stormbreaker was interchanged with Mjølnir between Thor and Captain America. What happened to Stormbreaker after Thor left Earth is unknown.

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 would crumble as your minds collapsed into 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 summon the 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. 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 was meant to be the strongest weapon in Asgard's history. As such, it is one of the few able to grievously wound Thanos, who faced off against the combined might of Doctor Strange, Iron Man, Spider-man, Drax, Star-Lord, Mantis, and Nebula while suffering nothing more than a tiny scratch on the cheek. Thanos himself mocked Thor for not aiming for his head, indicating Stormbreaker would have been capable of killing him instantly. Being built to stand against the Infinity Gauntlet-wielding Mad Titan, Eitri enchanted it with several properties that allowed it to take advantage of the weaknesses Eitri had installed into the Gauntlet in order to overpower it, quickly slicing through the beam unleashed by Thanos, although Thanos was caught off-guard and could have repelled the attack by other ways.

Trivia

 * In the comics, the Stormbreaker was created by Odin as a weapon for Beta Ray Bill, after he returned Mjølnir to Thor.
 * The Marvel Cinematic Universe 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 was able to defeat the Infinity Gauntlet.
 * In a Q&A with Collider, Joe and Anthony Russo answered a question regarding how Stormbreaker was able to take the brunt of six Infinity Stones, replying that it was the greatest weapon ever made and Thanos didn't know what that weapon was and didn't think he knew what he was up against in that moment.
 * When asked if Stormbreaker was more powerful than the Infinity Gauntlet, Joe and Anthony Russo responded that it certainly has the ability to counteract the powers of the Gauntlet but the key moment was that Thanos was caught off-guard; he only realized what that weapon was after he got struck by it as it came out of nowhere.