"Thor Slaying Hrungnir" by Ludwig Pietsch (1865)
Hrungnir ("The Brawler"[1]) was the mightiest of all of the giants, the spirits of darkness, winter, night, and the grave, who are often the enemies of the gods.
One day Hrungnir was paid a visit in Jotunheim, the homeland of the giants, by Odin. Hrungnir didn't recognize the god at first, and instead wondered aloud who this stranger might be whose horse could ride through the air and the water, as he had seen the horse do at the god's approach. Odin bet his head that his horse – none other than the eight-legged shamanic steed Sleipnir – could outrun any horse in Jotunheim. Hrungnir was insulted by this provocation, and straightaway accepted the bet and mounted his own horse, Gullfaxi ("Golden-Mane").
The two raced through mud and streams, over steep, rocky hills, and between the trees in thick woodlands. Before the giant realized it, he had passed through the gates of Asgard, the home of the gods. And, of course, he still hadn't caught up with Odin and Sleipnir. The gods, seemingly in good cheer, invited him to drink with them.
After he had become drunk, he became belligerent, and boasted that he would kill all of the gods except for the Freya and Sif, the wife of Thor. These two lovely goddesses he would carry back to Jotunheim with him. Freya alone was stout of heart enough to continue filling his horn. Next he bellowed that he would drink every last drop of the gods' ale. The gods soon grew tired of his anger and sent for Thor, who had been elsewhere fighting other giants.
When Thor arrived and discovered the situation, he lifted his hammer and prepared to slay Hrungnir there on the spot. The bellicose (and yet, we may suspect, inwardly fearful) giant accused Thor of cowardice for intending to kill someone who was himself unarmed. "Your name would be held in far higher honor," the giant declared, "if you will accept my challenge to a duel." Never one to lose an opportunity to gain renown and prove his abilities, Thor accepted.
When the arranged time had arrived, Hrungnir walked to the field near Jotunheim where the duel was to be held. He wore stone armor, brandished a stone shield, and menacingly waved a whetstone, his chosen weapon, in the air above him. Suddenly, he saw lightning and heard thunder clap above him, and Thor roared onto the battlefield. Thor hurled his hammer at the giant, and the giant slung his whetstone at the god. The stone burst against Thor's forehead and shattered into pieces, and this is the origin of all flint on earth. Thor's hammer also struck Hrungnir's head, but this time it was the giant's head that was shattered.
But a piece of Hrungnir's whetstone was lodged in Thor's forehead. So Thor sought out the sorceress Groa ("Thriving"[2]), who sang spells over the stone to remove it from the god's brow. When Thor felt the stone moving, he told the sorceress many joyous things to encourage her, chiefly that he had encountered her lost husband, who would soon be home. But Groa was so overcome with emotion upon hearing this that she forgot her chants, and the rock remained lodged in Thor's face until his death at Ragnarok.[3]
[1] Simek, Rudolf. 1993. Dictionary of Northern Mythology. Translated by Angela Hall. p. 161.
[2] Ibid. p. 120.
[3] Snorri Sturluson. The Prose Edda. Skáldskaparmál 24-25.