The Massacre of the Dwarves was an immense slaughter orchestrated by Thanos to prevent any future forging of weaponry on Nidavellir. Thanos obtained his gauntlet from Eitri and then he proceeded to wipe out all the Dwarves, which later resulted in the neutron star at Nidavellir's core burning itself of all its energy.
In an attempt to wipe out half of all life in the universe, the Titan Thanos sought to harness the power of the Infinity Stones, six concentrated ingots which held dominion over essential aspects of the universe. In order to use their powers, Thanos needed to have a device created for him which could exploit each Stone as he wanted. Knowing about the mastery of the Dwarves in craftsmanship and blacksmithing, Thanos headed to Nidavellir, home of the Dwarves.
Thanos went to Nidavellir, demanding Eitri to make a device capable of harassing the power of the Infinity Stones. Fearing for the lives of his fellow dwarfs and believing they would be safe if he did what Thanos demanded, Eitri successfully made the Infinity Gauntlet and Thanos proceeded to put it on and was finally ready to act on his quest to gather all six stones. Having successfully obtained the gauntlet, Thanos proceeded to massacre every dwarf, leaving only Eitri alive and told him that his hands were his alone and proceeded to personally cripple the Dwarf King's hands with metal coatings to prevent any further weapons forging.
In 2018, Thor returned to Nidavellir after Thanos slaughtered half of his people. As soon as Thor, Rocket Raccoon, and Groot arrived inside, Eitri attacked them. Thor calms Eitri down and asked what happened and what he did. Eitri sat down and told Thor that if he did what Thanos ask his people would be safe; however, they were all killed anyway. Thor tells Eitri that not all hope is lost and that every weapon he designed was not by his hands but all in his head. Thor asked Eitri to make a weapon to kill Thanos. Eitri then showed a piece of mold that in theory can summon the Bifrost, Thor then asked if it had a name, Eitri told him that it was known as Stormbreaker.