Gungnir is a three pronged spear made of the indestructible Asgardian metal Uru. Various stories are told about its origin and they are all different, but one point which most agree on is that Gungnir is incredibly ancient. One potential story is that Gungnir was a gift from dwarves who shaped it out of sunlight, then presented to the ruler of Asgard, Odin Borson. A second possible beginning of Gungnir tells that Loki, son of Odin and brother to Thor, organized a competition, to which the Sons of Invalid created Gungnir along with numerous other items. A third tale for the inception of Gungnir's creation tells that the prehistoric sphere is so old that it is rumored to have survived the destruction of earlier Asgard's which were obliterated as a result of Ragnarok. Which of these three accounts is true is still unknown.
Odin has used Gungnir throughout his lives, during battle, and while ruling as king of Asgard. Odin has also placed multiple enchantments on the spear, one of which is, that it returns to his hand after being thrown much like Thor's hammer, Mjolnir. This can be achieved with mental or verbal commands. Another assumed benefit of those enchantments placed upon Gungnir was that the spear was unable to be lifted by anyone other than Odin, again a similarity shared by Mjolnir. However, this was later shown to be false, anyone can handle Gungnir, though not to the capacity of Odin.
Gungnir is an object from norse mythology.
For full story arc information see: Siege
During the events of Siege, Norman Osborn orders the Thunderbolts to steal Gungnir from the armory on Asgard while his forces are laying siege to the realm. Osborn desires the powerful weapon to take control of Asgard, and potentially kill Thor. The Thunderbolts do obtain the spear of Odin but are interrupted by The Mighty Avengers, this results in the extremity's on the left side of U.S.Agent's body being cut off using Gungnir. The Thunderbolts win the battle against The Mighty Avengers and Gungnir comes into the possession of Paladin and Ant-Man, and eventually Paladin goes into hiding with the spear, with everyone on multiple worlds hunting him down for the legendary weapon of Odin.
The arc ends with Gungnir still hidden away.
Sir Anthony Hopkins potrays Odin in the 2011 movie, Thor. Gungnir appears throughout the movie, first in the possession of Odin as he uses it to battle Frost Giants, blasting them away with energy from the spear and blinding light released as well. Then when Frost Giants infiltrated beneath Asgard to take back the Casket of Ancient Winters, the source of their power, Odin uses Gungnir to "activate" the Destroyer and destroy the Frost Giants.
After Odin falls into the Odinsleep, Loki takes control of Gungnir, using the spear to rule Asgard in place of his adopted father. Sometime later Loki uses Gungnir to destroy the leader of the Frost Giants, Laufey, and a small band of his "men" that Loki tricked into thinking they could kill Odin while he slept.
Next, after Thor returns from Earth to Asgard and discovers Loki's treachery, the two brothers use their mystical items to do battle. Thor manages to knock Gungnir out of Loki's hands and subdue him. The spear finds its way back into the hands of Odin after he wakes from the Odinsleep and takes back his place as king of Asgard.
Thor: The Dark World
Gungnir will appear once again in the upcoming 2013 film Thor: The Dark World, sequel to Thor. Sir Anthony Hopkins returns as Odin.