Mjolnir (literally, "that which smashes") is the hammer wielded by Thor Odinson the Norse god of thunder.
Odin, chief of the Norse gods, ordered the Dwarfs to forge Mjolnir. The blacksmiths Eitri, Brok, and Buri used the core of a star as the mold. In the process the star exploded and almost took Midgard (Earth) with it; that event may explain the extinction of the dinosaurs, about 65 million years ago. Although powerful in and of itself, the hammer also received several potent spells from Odin, who slew the frost giant Laufey with it. Much later, Mjolnir was bequeathed to Thor, after he proved himself worthy through several trials.
With few exceptions, Thor has carried Mjolnir throughout his adventures. Thus, the history of Mjolnir is the history of Thor. Although Asgard was destroyed in Ragnarok, Thor, and thus the hammer, survived.
Mjolnir itself is already extremely durable and nigh invulnerable like the Earth metals such as Adamantium, Vibrainium or an alloy or even the Olympian metal Adamantine, and combined with the various enchantments placed upon it by Odin, is for all intents and purposes, indestructible. It has survived heat as extreme as the heart of suns, blasts powerful enough to destroy planets, and it has contained energy sufficient enough to destroy an entire galaxy and even energy sufficient enough to destroy one fifth of the Universe. Mjolnir also grants its user, such as Thor, countless mystical attributes and powers such as manipulation of the fundamental forces of the Universe such as the electromagnetic spectrum, gravity etc. Mjolnir can be used both offensively and defensively. Thor often uses the hammer as a physical weapon, with almost nothing being capable of withstanding a hammer blow or throw. It has been described as impacting with sufficient force to "destroy mountains" and has proven capable of shattering the armor of a Celestial. A rare exception is primary adamantium.