Mjølnir

"Whoever wields this hammer, if he be worthy, shall possess the power of Thor."

- Odin

Mjølnir is Thor's powerful Asgardian enchanted war-hammer.

History
"Sir. We found it."

- Phil Coulson

Mjølnir was forged from the heart of a dying star. It was given to the Asdgardian prince Thor by his father, Odin, the King of Asgard.

When Thor and his friends, Sif and the Warriors Three, attacked Jotunheim, destroying the fragile truce between the Asgardians and the Frost Giants, Odin punished him by stripping him of his godly powers. Odin exiled Thor to Earth as a mortal, accompanied by his hammer Mjølnir, now protected by an enchantment Odin spoke into it that allows only the worthy to wield it. Mjølnir ended up in New Mexico.

When Agent Phil Coulson arrived to New Mexico, he observed a massive crater created by Mjølnir. He then called Nick Fury to inform him that he found it.

When Thor discovered Mjølnir's nearby location, he attempted to retrieve it from the facility that S.H.I.E.L.D. quickly constructed but he found himself unable to lift it, and was captured. With the help of Erik Selvig, he was freed and resigned himself to exile on Earth. Later, Loki, invisible to mortals, came to the facility where he attempted to lift the hammer, but he was unable to. When he seized the throne of Asgard, he sent the Destroyer to Earth to kill Sif, the Warriors Three, and Thor. The Destroyer attacked and defeated the Asgardian warriors, prompting Thor to sacrifice himself instead. Struck by the Destroyer and near death, Thor's sacrifice proved him worthy to wield Mjølnir. The hammer returned to him, restoring his powers and enabling him to defeat the giant automaton.

Thor returned to Asgard where he stopped Loki's plan to destroy Jotunheim. To do that, he had to destroy the Bifrost Bridge with Mjølnir.

Some time later, when Loki reappeared on Earth, Thor came to capture him. Loki was captured by Iron Man and Captain America, but they weren't willing to hand him over to Thor. Thor fought against them, but even Mjølnir was powerless against Captain America's shield.

Abilities
Thor has been entrusted with the mighty hammer Mjølnir, that was forged from the heart of a dying star. Thor's mystical hammer Mjølnir, which resembles a mallet rather than a war hammer, has a number of elemental powers. It has been stated by Odin himself that Mjølnir's power has no equal. Mjølnir itself is extremely durable like the Earth metal vibranium, and, combined with the various enchantments placed upon it by Odin, it is, for all intents and purposes, indestructible. It has deflected three blast from the Destroyer (which was able to disintegrate anything that it hit) and return the blast back into the Destroyer. Thor often uses the hammer as a physical weapon, with almost nothing being capable of withstanding a hammer blow or throw (with the exception of Captain America's Shield). Thor used Mjølnir for a variety of short range and long range attacks either by direct hits or throwing it. It returned to Thor whenever he threw it or summoned it, regardless of the impending obstacles and distance; even traveling through realms. Whenever dropped by Thor, it assumed a fixed position from which no one could move it except a "worthy" individual. Thor also used it for directing his summoned elements such as lightning, wind, rain, and snow. Odin stated "its power has no equal. It is a tool to build or a weapon to destroy. It is a fitting companion for a king." A few examples of the abilities Thor has shown with Mjølnir are:
 * Weather Control: Wielding Mjølnir grants Thor the ability to control the base elements of a storm. It can control the elements and can create giant raging electrical storms complete with thunder, lightning, hurricane-force winds, tornadoes, tidal waves, earthquakes, and torrential rains on a moment's notice. He summoned a class 5 tornado to battle the Destroyer, a lightning bolt strong enough to destroy a large piece of landscape in Jotunheim, a lightning bolt to knock down Loki on the Bifrost bridge, and control the elements. He was able to strike Iron Man with a bolt of lighting, but it is notable that all it did was charge his suit with additional power. He was able to strike Chitauri soldiers down and Leviathans. The Norse myths and legends regarded him as a Thunder God because of the hammer's influence over weather.
 * Lightning: Thor's main weapon using Mjølnir is his lightning bolt or thunderbolt. He summoned a lightning bolt strong enough to destroy a large piece of landscape in Jotunheim, a lightning bolt to knock down Loki on the Bifrost bridge, shot a lightning bolt at Iron Man that boosted his armor's power reserve to +400%. Thor shot a massive lightning bolt at the portal of the Chitauri to prevent anymore reinforcements, took down a Chitauri warship with a single thunderbolt.
 * Worthiness Enchantment: After Thor disobeyed Odin and nearly caused a war between Asgard and Jotunheim, Odin banished Thor to Earth without his powers and placed an enchantment on the hammer Mjølnir. "Whosoever holds this hammer, if he be worthy, shall possess the power of Thor." This enchantment surrounding Mjølnir prevents it from being wielded by anyone save those who have been found worthy. When Thor sacrificed himself to protect Earth from The Destroyer he proved himself worthy to wield Mjølnir once again and gained full use of his powers. Only Thor and Odin are able to wield Mjølnir. Other than that, none of the citizens of Puente Antiguo or Loki could lift it. Not even the Hulk's incredible strength could lift Mjølnir.
 * Material Transformation: By summoning lightning from the sky into Mjølnir, Thor's uniform was transformed into his battle armor.
 * Mystical Link: Mjølnir obeys Thor's commands as though it were alive. Mjølnir went to Thor when it was in a crater from miles away, it came to him in seconds. Mjølnir will return to Thor whenever he throws it at an opponent. While battling the Hulk in a hanger of the Helicarrier, Thor summoned Mjølnir from another room and it came crashing through the Helicarrier to get to him.
 * Flight: Thor is capable of hurling Mjølnir with great force and, by holding onto the leather thong, is capable of flying through the air at tremendous speeds. How fast is not specified, but he was able to remain aloft in the middle of a Class 5 tornado, and traverse the distance from his father's chambers to Heimdall's observatory and back again in seconds. He used Mjølnir to fly into combat against Iron Man in the woods, to fly to the top of a skyscraper and on top of a Chitauri warship.
 * Speeds: Thor is capable of hurling Mjolnir at great speeds by holding onto the leather thong. In Jotunheim, he bashes the Frost Giants around him, then throws Mjølnir, making it hit multiple Frost Giants in it's path in moments.
 * Energy Projection: With Mjølnir, Thor can project powerful mystical blasts of energy. He can even channel energies for stronger energy attacks. With Mjølnir, Odin projects a powerful mystical blast of energy using it to channel an energy blast through it, taking away all of Thor's superhuman powers and destroys what was left of Thor's armor after stripping much of it off of him and throwing him into the Bifrost to Earth.