Jörmungandr's arch-enemy is the thunder-god, Thor, with the two being prophesied to kill each other come Ragnarök. Jormungandr will be slain by Thor, who will then take nine-steps away before succumbing to the serpents Eitr poison that it had been spewing in the air during the battle.

Why did Thor hate Jörmungandr?

And legend had it that they did kill each other in their final battle. If Odin and Fenrir became sworn enemies for Odin's cautions against Ragnarok, Thor hated Jormungand for his Midgard Serpent was the only one in the cosmos that could be a challenge for him.

What serpent did Thor kill?

Jörmungandr is the Midgard Serpent (also World Serpent) in Norse mythology who encircles the realm of Midgard. He is the son of the god Loki and the giantess Angrboða and brother of the great wolf Fenrir and Hel, Queen of the Dead. At Ragnarök, the Twilight of the Gods, he slays and is slain by the god Thor.