The strongest god has two sons: Magni („the mighty") and Modi („the brave / wrathful"). They are more than minor figures – for they are among the few who survive the end of the world.
Magni is the son of Thor and the giantess Járnsaxa. When Thor slays the giant Hrungnir, the giant’s huge leg falls on the stunned god, and none of the Æsir can lift it. Then the barely three-night-old Magni comes and lifts the leg aside with ease – saying that had he been called sooner, he would have killed the giant with his fist. Thor’s strength lives on in him.
After Ragnarök, when the old divine world falls, Magni and Modi are among the survivors. They inherit the hammer Mjöllnir and carry it into the new, green world. So even after the world’s end the protecting power of the thunder-god endures.
“Modi and Magni shall wield Mjöllnir / when Vingnir falls in the fight.”— after Vafþrúðnismál 51, Karl Simrock (public domain)
Magni’s mother is the giantess Járnsaxa – a sign of how closely Æsir and giants are interwoven. His superhuman strength shows already in the cradle: only he can lift Hrungnir’s leg, which lies even on the stunned Thor’s neck.
The real reason these two sons matter, though, lies at the end of time. After Ragnarök a new, green world rises from the sea, and a handful of gods return: Baldr and Höd, Vidar and Vali – and Magni and Modi, who bear the hammer Mjöllnir. The protecting power of the thunder-god thus outlasts even the world’s end. That is the consoling keynote of Norse myth: after the end comes not nothing, but a new beginning.
Thor · Mjöllnir · Ragnarök · What comes after.
Snorri, Skáldskaparmál (17); Vafþrúðnismál 51 (Poetic Edda). Rudolf Simek, Dictionary of Northern Mythology.