The Jörmungandr is the overarching antagonist of the animated television series Dragons: The Nine Realms and the main antagonist of the 8th and final season overall.
It is also known as the World Serpent and fueled the tale of the the Midgard Serpent from ancient viking times.
Jörmungandr is a dragon that Hiccup Horrendous Haddock III defeated prior to the series. Toothless presumably made sure that none had ever released the serpent as it was the apex predator of the dragons. It was locked in a specialized cage created in the Dark Realm that can only be opened by the muzzle of a Night Fury dragon or a descendant Night light. It is known as the apex predator to all dragons.
Weapons: Inferno, Astrid Hofferson's Axe, Hiccup's Viking Helmet, Hiccup's Shield, The Mangler (Hiccup's Bola Catapult), Dragon Root Arrows, Toothless' Saddle and Tail Fin, Dragon Scale Armor, Deathgripper Venom
Plants: Dragon Root, Blue Oleanders, Purple Oleanders Other: Yak Nog, Bork's Papers, The Book of Dragons (original, The Nine Realms), Hiccup's Prosthetic Leg, Astrid's Betrothal Necklace, Gobber's Peg Leg