Jörmungandr, or also known as Serpent of the World, is a serpentine Beast Jötunn and one of Loki's children in Norse mythology as well as a major player in the events of Ragnarök, which was the Norse version of the End Times (though unlike monothestic faiths Ragnarok was seen as the end of a cycle rather than time itself, with a new world destined to be born from the old world's destruction).
Jörmungandr was one of the apocalyptic beasts who would lay waste to the current world and help in killing off the current gods (known as the Æsir), though would also be slain in the process, allowing for new life to form from the ashes of the old. The son of Loki and Angerboda, a giantess who was counted among his wives, Jörmungandr was unfortunate enough to be born as a Beast Jötunn like his brother Fenrir due to being born with monster form where in his case, a draconic serpent. As with his canine brother, he was banished by Odin to the depths of the ocean in Midgard due to the threat he pose on the Gods as told in the prophecy. Though this act can be considered as self-fulfilling since Jörmungandr would eventually fulfill the prophecy of Ragnarök regardless, it can be argued that the Gods of Asgard hardly having alternative given that like Loki, Jörmungandr inherited his kind's burning hatred towards them for their murder on Ymir, a hatred than not even wisest of Gods of Asgard can quell.
The prophetess Volla prophecized that at the time of Ragnarok, the Midgard Serpent would be slain by Thor in battle, but then Thor himself would perish by the serpent's venom.
While in the court of Utgard-Loki, Thor was challenged to lift his cat off the ground, but was humbled when the cat managed to keep at least one foot on the ground at all times. Utgard-Loki then revealed his so-called cat was actually the Midgard Serpent in disguise.
Thor went fishing with an ox-head on the boat of the Jötunn Hymir for the Midgard Serpent. Thor succeeded in catching the serpent, but Hymir grew afraid and cut the line believing he was saving Thor from himself. As the serpent retreated, Thor grew angry with Hymir and struck him. He then departed under the power of his hammer as he told Hymir he ought to be grateful that he was letting him live.
Although not much is known about this serpentine Jötunn's personality, the Gods of Asgard correctly deduced that Jörmungandr didn't fare better than his lupine brother Fenrir that they casted him to the ocean in Midgard straight away. Because of said deduction was stemmed from a prophecy that surrounding his role in Ragnarök, it can be inferred that the Gods themselves ironically have part in shaping Jörmungandr into the very apocalyptic beast he eventually become, making it a rather self-fulfilling prophecy. Taking into accounts of Jötnar and those who related to them by blood mostly harbor strong, genocidal hatred against the Gods since their murder on Ymir and Fenrir already troubled them as much as their father Loki on the other hand, the Gods of Asgard may hardly had alternatives at that time and opted to do so to delay the worst, making such decision justified at a very least.
Jormungandr is a popular Norse monster, though ranking below his siblings Fenrir or Hela as well as his infamous father Loki — nevertheless the "World Serpent" is a common and frequently seen antagonist in many modern takes on the Norse myths, some of the most popular of which include:
Marvel Comics, who adapted Thor into a superhero franchise, has depicted Jormungandr as a recurring foe to Thor - both in original comic book format and in more recent cartoons.
One of latest updates of For Honor game is introduction of a cult and hero unit named and themed after Jormungandr himself.
Jormungandr appears as one of playable characters in online game SMITE.
Final Battle against Thor by Emil Doepler
Jormungandr with his siblings, Fenrir and Hela by Willy Pogany
Jörmungandr gets fished by an ox head, from the 17th century Icelandic manuscript