In the ancient days when the worlds of gods and giants were separated by more than just distance, the boundary between Asgard and Jotunheim was a place of constant tension and sudden violence. The story of Thor’s duel with Hrungnir begins not with a hammer blow, but with a wager of pride. Odin, the All-Father, was riding his eight-legged steed Sleipnir through the frozen wastes of Jotunheim. There he encountered the giant Hrungnir, who sat atop his magnificent gold-maned horse, Gullfaxi. Hrungnir was known as the strongest of the Jötnar, possessing a heart made of solid stone with three sharp corners, and a head equally as hard. When Odin boasted that no horse in Jotunheim could match Sleipnir, Hrungnir’s pride was stung. The giant leaped onto Gullfaxi and a wild race began. Across the frozen plains and over the mountain peaks of the Jotunheimen range they flew. So intense was the race that Hrungnir did not realize he had crossed the threshold of the Bifrost until he found himself within the very gates of Asgard.
Though he was an intruder, the laws of hospitality in the Viking world were sacred. The Aesir invited Hrungnir into their hall to drink. However, the giant’s temperament was as rocky as his physique. As he consumed the divine mead, his intoxication fueled a dangerous arrogance. He began to boast that he would pick up Valhalla and carry it back to Jotunheim, that he would sink Asgard into the sea, and that he would slaughter all the gods except for the beautiful goddesses Freyja and Sif, whom he intended to keep for himself. The gods, weary of his insults and fearing his mounting rage, called for Thor. The God of Thunder had been away in the east hunting trolls, but he returned with a crash of lightning that shook the foundations of the hall. Thor was not known for his patience with giants, and he immediately raised Mjolnir to strike. However, Hrungnir claimed the protection of the guest-right and challenged Thor to a formal duel at Grjotunagardar, on the border of their realms. To strike an unarmed guest would have been a stain on Thor’s honor, so the challenge was accepted.
When the giants in Jotunheim heard of the upcoming duel, they were seized by a cold dread. Hrungnir was their strongest champion; if he fell, the Aesir would have little reason to fear the Jötnar. To tip the scales, they constructed a monster out of clay at Grjotunagardar. This clay giant, named Mokkurkalfi, stood nine miles high and was wide across the chest. Because they could find no human heart large enough for such a creature, they gave it the heart of a mare. Despite his size, Mokkurkalfi was a coward at heart, his legs trembling at the mere thought of the Thunder God’s arrival. Hrungnir, meanwhile, prepared himself with his stone shield and his primary weapon: a massive, sharp-edged whetstone used for sharpening tools, which he wielded like a club.
On the day of the duel, Thor arrived in a storm of fury. He was accompanied by his swift-footed servant, Thjalfi. As they approached, Thjalfi ran ahead to where Hrungnir stood. In a cunning move of psychological warfare, Thjalfi shouted to the giant that Thor was planning to attack him from underground, surging up through the earth. Hrungnir, believing the trick, placed his stone shield beneath his feet and stood upon it, leaving his upper body exposed and holding his whetstone with both hands. This was the moment Thor appeared in the sky, heralded by the roar of thunder and the blinding flash of lightning. Thor did not attack from below; he swung Mjolnir from afar, the hammer screaming through the air like a falling star. Hrungnir reacted instantly, throwing his heavy whetstone at the approaching hammer. The two weapons collided in mid-air with a sound that cracked the very sky. The whetstone shattered into pieces. One fragment fell to the earth, forming the flint mountains found across Midgard, but another sharp shard lodged itself deep into Thor’s forehead, causing the god to fall from his chariot.