Of the three monstrous children of Loki and the giantess Angrboða, Jörmungandr is the middle one — wolf-brother to Fenrir, half-brother to Hel, and the serpent the gods feared most quietly. When Odin saw the prophecy attached to the children, he had Fenrir bound, Hel sent to rule the dead, and Jörmungandr hurled into the great ocean that surrounds Midgard, the world of men.
He grew there. He kept growing. The Norse called him Miðgarðsormr — the Midgard Serpent — because in time he became so vast that he circled the entire world beneath the waves and could bite his own tail. The Eddas are very specific about it: as long as Jörmungandr holds his tail in his mouth, the world stays whole. On the day he lets go, Ragnarök begins.
He has one great enemy, and they meet three times. The first time, the giant-king Útgarða-Loki disguises Jörmungandr as an enormous grey cat and dares Thor to lift it. Thor, not knowing what he is wrestling, manages to raise a single paw off the floor — which lifts the serpent's coils clear of the seabed and nearly cracks the world open. The giants are terrified. Even Thor's failure is a kind of victory.
The second time, Thor goes fishing with the giant Hymir, baits his hook with an ox-head, and hauls the serpent up out of the deep. The two glare at each other across the gunwale. Thor raises his hammer. Hymir panics and cuts the line — and the serpent sinks back into the ocean. The Norse storytellers were torn for centuries about whether Thor would have won that day if Hymir had held his nerve.
The third meeting is the last. At Ragnarök, Jörmungandr finally releases his tail and rises from the sea, spitting a flood of venom that poisons the sky. Thor meets him on the shore. He kills the serpent with his hammer — and then walks exactly nine paces before the venom kills him too. They end together, as it was always going to end.
Wear Jörmungandr and you wear the Viking shape of the world itself — the serpent in the deep who holds the circle closed, and the long, patient knowledge that one day every circle opens.