Gungnir is a spear and a weapon that belongs to Odin the chief of the Norse deities. In legend, the spear was fashioned by the Dwarves; Loki discovers the Spear whilst visiting the Dwarves, although it is said to be on an errand to commission golden hair for Sif.
At first the spear was stated to have no power on its own. It served as a natural channel for Odin's own power in the form of energy blasts and other effects. It would obey Odin's command to mentally return to him and could only be wielded by Odin. Later stories stated the spear was equal to Mjolnir in power. If thrown it would fly after its target and its tip would be surrounded with electricity. It could destroy towers with ease and would return to its owner when thrown.
Loki flatters the Dwarves and asks the Spear of them, which they give him. In the Poetic Edda poem, the Aesir-Vanir War is described as officially starting when Odin throws a spear over the heads of an assembly of Vanir gods. Whether or not this was specifically Gungnir is, however, unstated. It is said that during Ragnarök Odin will use the Spear in battle in order to fight the monster wolf Fenrir.