Magni was born roughly 20 years after Thor and the other Asgardians took control of Earth. As an infant, he was taken hostage by Balder during a revolt by the remnants of Earth's heroes. Loki killed Balder and Thor put an end to the heroes.
As he grew up, Magni adventured throughout the kingdom, battling the Midgard Serpent, averting natural disasters and other adventures. He grew to dislike Loki's treatment of the humans but followed his father's wishes out of loyalty. This doubt deepened further when he, the Warriors Three and Thialfi investigated a rebellion witnessing the cruel brutality of the asgardians and the sheer amount of resentment towards their occupancy of earth displayed by the mortalities.
Magni had also entered a relationship with a human woman named Jordahl, a petty thief and pickpocket whose free spirited fiery persona perked his interests. When Loki found out said common criminal had caught the princes eye after she'd eluded capture, he impersonated him to resume their get together only have both her and her mother imprisoned, taken to one of his reconditioning facilities. After Magni learned of what had happened from local residents after he'd paid visit to her ruined home, he flew into the internment camp and rescued her eventually taking the young lass to an old friend. While communing with the exiled Lady Sif in order to help her he found himself at a loss, full of lamentation over the erasure of her once vibrant persona.
When he returned home only to witness that Kya being hanged, her spirit led him to Mjolnir, which he would take the time and experiment with in discovering its usage. Magni went to see Sif again after finding he was chosen worthy of the fabled hammer of his father, under her tutelage in its use they ventured to Asgard only to have learned from a withered Ulik, that it had fallen into decay due to lack his peoples presence there. Armed with this new knowledge the two returned home and confronted Thor.
As he challenged the All Father to lift the sacred mallet after much refutal by his birth mother and conniving uncle after accusing them both of being power mongering toadies feeding into their lords faulty leadership, they were interrupted when New Asgard came under siege by the return of Desak. Magni seemed unsurprised by this turn of events just as his father was poised to accept his sons chap, though if knowing the slayer of god's arrival was somehow a karmic reckoning for them, once again rebuked by Thor for his claim Magni sets out to battle the unassailable threat beside him. Though injured in the ensuing battle The Son of Thor pressures his attack repeatedly, staying in the fight just long enough to watch his uncle unleash The Destroyer upon their foe. To their horror however the host powering the destroyer was revealed to be the scourge of deities' creator; The Designate.
Both father and son partake to Loki's misdeeds in keeping her from uplifting mankind to keep them beneath Asgard's collective boot before having the battle resumed for them, yet after dealing what was presumed to be a finishing blow. Terene's agent quickly recovered while the cosmic entity herself put Magni down for the count indefinitely. Thor finally realizing the consequences of his actions, undid the events leading to this future going back in time and blending his divine and moral consciousnesses back together again. 
Magni seemed to possess the same power and skill as his father Thor though to a greater degree physical wise due to being God of Strength.
Magni could lift in excess of 100 tons.
Before he wielded Mjolnir, Magni used axes.
- Magni was the first to be worthy and pick Mjolnir up after his father Thor, in an act of rage, killed his former alter ego Jake Olson, and was forced to abandon it.
- Magni is apparently based on the character of the same name in Norse Legend. However in the legends his mother is a giantess named Jarnsaxa.