Hrungnir (Old Norse "brawler") was a jötunn in Norse mythology, slain by the god Thor with his hammer Mjölnir. The account is documented in the Skáldskaparmál, in the Prose Edda by Snorri Sturluson.
Prior to his demise, Hrungnir engaged in a wager with Odin in which Odin stakes his head on his horse, Sleipnir, being faster than Hrungnir's steed Gullfaxi. During the race, which Sleipnir wins, Hrungnir enters Valhalla, and there becomes drunk and abusive. After they grow weary of him, the gods call on Thor to battle Hrungnir.
The Fight and Enchantment
Thor and his servant Thjalfi challenge the giant, who hurls his whetstone weapon at Thor. Smashed to smithereens by Thor's hammer Mjölnir, fragments of the whetstone fall down to earth - and "thence... come all the flint-rocks" - while one shard sinks deep into the god's forehead. Nevertheless, the hammer strikes Hrungnir dead, shattering his skull; but in his fall, Hrungnir's dead body topples over Thor, leaving the god buried under one of his legs.
When both Thjalfi and the combined strength of the Æsir fail at pushing and pulling the giant's foot off Thor's throat, Magni, Thor's infant son with the giantess Jarnsaxa, passes by and easily lifts the foot, rebuking his father for his weakness. Back in Asgard, the sorceress Groa is called upon to remove Hrungnir's whetstone from Thor's forehead. As her enchantments are beginning to show an effect, gradually loosening the stone, Thor promises to generously reward her for her services, mentioning that he had recently helped her husband Aurvandil cross the icy river Eliwagar and that it would not be long for her to be reunited with him. Rejoicing at this news, Groa, in her excitement, forgets all about her chants, thus leaving the whetstone locked in Thor's forehead.