Nidavellir is a dying neutron star and one of the Nine Realms. Its core is orbited by a multi-ringed megastructure which served as the home world of the Dwarves.
Nidavellir was the home of the Dwarves, and one of their main activities is to serve as blacksmiths. As such, Nidavellir presents some of the best forges in the Nine Realms. It was here where they created the hammer Mjølnir.
In a plot to overthrow Odin as king of Asgard, the Asgardian Imir invaded Nidavellir, and barricaded himself in the Golden Forge. There, he took the Dwarven Smiths as hostages and made them forge an Axe capable of cutting through the fabric of reality.
When Thor arrived at Nidavellir, he confronted Imir, but he was taken by surprise by the powers of Imir's Axe, that allowed him to open small portals and teleport himself through the room, catching Thor off-guard until he was able to summon lightning through the chimney of the forge, electrocuting Imir and liberating the Dwarves.
When the Bifrost Bridge was destroyed, the Rock Trolls, having coveted the Dwarven Forges for many millennia, attacked Nidavellir, knowing that Asgard could not come to defend King Eitri and his people.
During the conflict with Thanos, Thor ventured to Nidavellir with Rocket Raccoon and 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, creating Stormbreaker with a handle made from Groot's arm. The then smoking husk of Thor was revived and used the power of the Bifrost to go forth to Wakanda and assist his fellow Avengers.