In order to avoid being crushed by his brother Thor, for cutting cutting Sif, Thor’s wife’s hair, Loki is sent to the dwarves forges in Svartalfheim, to get a new head of hair made for Sif. 
 
Deep in the caves, Loki tasks The Sons of Ivaldi to forge Sif’s new hair. The craftsmen not only complete that task, but Loki has them build the best ship ever made, as well as the deadliest spear. 
 
Loki is not done in the cave forges, he shows the  Ivaldi creations to another family of dwarf blacksmiths, and wagers his own head that they cannot create three equal, marvels. The dwarf brothers Brokkr and Sindri waste no time creating there first of three marvels a golden haired boar, that that gives light, and can run anywhere, even over water, or in the sky. Even while Loki tries to sabotage each creation disguised as a pesky biting fly, in order to save his head, the brothers forge their second treasure a golden ring, the ring’s magic allows it to spawn eight rings, on every ninth night, of the exact same size. 
 
Now Loki has to work even harder to stop the dwarves on their final work, he arrives at the bellows once again as a fly and instead of biting Brokkr’s arm or neck, as he did the previous nights, Loki bites Brokkr’s eyelid so hard, he has to wipe blood from his eye, stopping the bellows just long enough that his brother Sindri’s final creation comes out incomplete. Mjölnir the hammer emerges from the forge with a shorter than expected handle, but still holds enough power to destroy mountains, and create thunderous storms. 
 
 
Mjölnir is given to Thor, his wife her new hair, Odin his ring and spear, and to Freyr the ship and boar. Loki through his cunning tricks ends up keeping his head, and the mighty God Thor now has a weapon that nobody can wield but him.