Mjølnir ("that which smashes")[1] was a powerful enchanted war-hammer forged by the Dwarves for the Asgardian Royal Family. The hammer was destroyed by Hela during a confrontation between her, Thor and Loki in 2017.
Following the Snap, Thor time traveled to 2013 and brought the past version of the hammer into 2023, where it was primarily used by Captain America during the Battle of Earth. Following Thanos' death, Captain America traveled back in time to Asgard of 2013 in order to return Mjølnir.