Stormbreaker

"Does it have a name?" "Stormbreaker." "Eh, it’s a bit much."

- Thor, Eitri and Rocket Raccoon

Stormbreaker is an enchanted maul 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
During the conflict with Thanos, Thor ventured to Nidavellir with Rocket Raccoon and adolescent Groot to forge Thor's new weapon. 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 mould. He then shattered the mould, 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 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 (causing Groot considerable pain) 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 noticable harm to Thanos during the entire conflict; Thor impaled Thanos using it just after Thanos completed the Infinity Gauntlet, though while it was a severe injury, it was not fatal and Thor was unable to prevent Thanos from snapping his fingers, causing the Battle of Wakanda to be nevertheless lost to the Avengers.

Capabilities
"It can even channel the Bifrost."

- Eitri to 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, even wounding Thanos. It is also able to channel dimensional energy and act as a Bifrost Bridge, which is shown when Thor teleports himself, Rocket Raccoon and Groot to Wakanda instantly.

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 an ax head and a hammer head 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.

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.