Magni was the son of Thor, god of thunder, and the giantess Járnsaxa, one of the god's lovers. As a child, Magni was exceptionally strong for his age. One such display of his strength was during his father's battle with the giant Hrungnir. Thor hit Hrungnir in the head with his hammer, Mjölnir, shattering Hrungnir's skull. The giant then fell dead, as his leg landed on Thor's neck, pinning the god to the ground. The other members of the Æsir tried to lift the leg off of Thor, but were unsuccessful. Magni, who was only three days old at the time, then came to his father and lifted Hrungnir's leg off of Thor all by himself. In gratitude, Thor gifted his son the horse Gullfaxi, which Hrungnir previously owned. Odin, however, was upset that Thor gave the horse to "the son of a giantess" and not to Odin, Thor's father.
- His name Magni means "mighty".
- He is reputedly the strongest god in Norse mythology, even more than his father, Thor.
- Magni, alongside his brother Móði, is fated to survive Ragnarök, after which they come across their father's hammer, Mjölnir. They will then sail to the field of Iðavöllr, where Asgard once lay, whereupon they will meet with their uncles, Víðarr and Váli.