Then the giants made a man of clay at Grjótúnagard: he was nine miles high and three broad under the arm-pits; but they could get no heart big enough to fit him, until they took one from a mare. Even that was not steadfast within him, when Thor came. Hrungnir had the heart which is notorious, of hard stone and spiked with three corners, even as the written character is since formed, which men call Hrungnir's Heart. His head also was of stone; his shield too was stone, wide and thick, and he had the shield before him when he stood at Grjótúnagard and waited for Thor. Moreover he had a hone for a weapon, and brandished it over his shoulders, and he was not a pretty sight. At one side of him stood the clay giant, which was called Mökkurkálfi: he was sore afraid, and it is said that he wet himself when he saw Thor.
After Thor kills Hrungnir with his hammer, Thor's servant Þjálfi attacks Mökkurkálfi and kills the clay giant with little glory.