According to the references of Akaviri, Altmeri, Bosmeri and Nordic ancient texts, Jörmunir was defeated during the conclusion of the Dragon War in the Merethic Era by Alduin, envious of the power Jörmunir held over him. He ordered his blood kin to be dishonorably buried beneath the great ocean that encircles modern day Solitude, never to be found.
"Erupted from the seas of Karth, arose Jörmunir. There it appeared as if stricken from the depths of Bormahu's realm itself; and behold a great serpent it was, bearing scales of ebony and teeth of a Divine, surrounded by an aura of vengeance forever flowing through its wings." - The Great Serpent
Jörmunir is a very arrogant and prideful dragon. Due to his extremely powerful pride, as well as his confidence, he has grown more cocky with every battle he has won - and he's won quite a few. And since fleeing the isle of Akavir after his brothers left, his pride has grown significantly. Despite being attacked by numerous Dragonguard hunters, Blades agents and other dragons, Jörmunir has few scars from their engagements. He openly boasts to those who try to hunt him, and does not fear any creature. This includes other dragons, and Jörmunir has no qualms about attacking and killing those of his race who offend or endanger him.
Jörmunir is an ebony-colored serpentine of great size. He is almost twice the size of comparable dragons, such as Paarthurnax and even Alduin. He has black eyes which transit to crimson when focused. His most noticeable feature is the aura which constantly encircles him, said to be the very essence of his hate against Alduin.