Jormungandr was the name of the Midgard Serpent. Jormungandr Midgard Serpent was the son of Loki and giantess Angrboda and the brother of Wolf Fenrir and the Underworld ruler Hel. So enormous was he that his body built up a circle surrounding the entirety of the Midgard (the land of human) which coined him the name the Midgard Serpent.
Just like two siblings of him, Jormungandr was consigned to another place as a prevention of Ragnarok. Actually, Odin the Allfather was the one that cursed Jormungandr to the dark ocean where he remained vulnerable and waited for the Ragnarok to slay the gods. According to the ominous prophecies, when Jormungandr released his tail from his mouth and raised above the ocean, there finally came the Ragnarok days.
Jormungandr Midgard Serpent and Thor
Jormungandr was the ancient mortal enemies of Thor. As the surviving materials depicted, Thor and Jormungandr encountered each other twice.
The first encounter was when Thor went fishing with a fierce giant named Hymir. As they were on the boat for fishing, an enormous fish caught the bait (the fish was Jormungandr Midgard Serpent actually). Thor knew there was something wrong with the fish and eventually realized that the fish was Jormungandr – his enemy. He tried to kill the serpent with his hammer the Mjolnir. Hymir was terrified of the serpent’s presence from the ocean and afraid this would be the harbinger of Ragnarok. The giant Hymir then cut the fishing line and released the serpent into the waves.
The second time they encountered each other, of course, was during the event of Ragnarok. After the serpent finally raised above the waves of the ocean, the war between the giant and gods began. Jormungandr teamed up with his father and brother to make their way to the Asgard. They killed all the beings that came up in their sight. Wolf Fenrir opened his mouth and used his sharp teeth to devour all the obstacles. Jormungandr poisoned the whole world with his terrible and deadly breath. Up in Asgard, Jormungandr fought with Thor. Thor the God of Thunder successfully finished the life of the Midgard Serpent. However, his left venom killed God Thor too.