Jörmungandr, the Midgard Serpent, is the second monstrous child of Loki and the giantess Angrboða. Odin, foreseeing the danger the three siblings posed, cast the serpent into the sea that surrounds Midgard, where it grew so vast that it now encircles the entire world and grips its own tail in its mouth. Its coils mark the boundary of the human world, and its movements are said to stir up storms.
Thor is its fated adversary, and the two clash memorably even before Ragnarök: disguised as a young man, Thor joins the giant Hymir on a fishing trip and baits his line with an ox’s head, hauling the serpent up from the deep and glaring into its poison-flecked eyes before the terrified Hymir cuts the line and lets it sink back into the sea.
drepr af móði Miðgarðs véurr, munu halir allir heimstöð ryðja; gengr fet níu Fjörgynjar burr neppr frá naðri níðs ókvíðnum
”… Nine paces fares the son of Fjorgyn, / And, slain by the serpent, fearless he sinks.” — Völuspá 56, tr. Bellows
Bellows glosses “the son of Fjorgyn” as Thor: the serpent appears here only as the agent of his death, which is as close as the Völuspá comes to naming Jörmungandr’s own end.
That unfinished duel is settled at Ragnarök: Thor at last kills Jörmungandr, but the serpent’s venom, spat out as it dies, fells him nine steps later. Of Loki’s three children by Angrboða, Jörmungandr’s story is the one most bound to Thor’s own, each existing in myth chiefly as the other’s destined end.
