So, a bit easy and predicted, but fun none the less. I have tried to slightly dodge the obvious and taken a slight variation on the myths.
Son of the famous world devouring serpent Jǫrmungandr. While his sire is sleeping he tries to avenge his grandsire by tricking the people on midgard (primal plane) to cuase suffering. He is often found near bodies of water. When Jǫrmungandr entered the water and grew ever larger, he begot a son with a water nymph. His son, bearing a likeness to both his parents, had a knack for controlling water. What Jormangand lacked in size he coudl compensate with his control of water and ability to take on other forms. Jormengand is often found near the watertrying to lure in people and gods, only to drown them in sudden whirlpools and currents. When he is really mad he uses waves to wash away anything on the coasts. When changed into another form snakes and serpents will be more active. When His fater was put to sleep by the gods
Jormangand is always searching for the realms of Hel to help free his grandsire, wake his father and bring about Ragnarök.
Beware the waters
All bodies of water grow in ferocity and serpents will start stirring
Faint: Springs, geishers, creeks and ponds will flow more. Springs will start gushing hteir water, geisers will spout higher and more explosive. Ponds will be darker and corrupt more matter. Creeks will start flowing quicker and grow in size. Snakes will start moving aboutmore and nearer to people.
Moderate: Rivers, seas and lakes will become unruly and/or have their flow increased. Also all waterbodies will contain smalls erpentssources of fresh water (ponds, wells) will spoil and decay. Swamps will spread and snakes and serpents will come slithering fout of them.
Strong: Seas and oceans will become untraversable and wild. Rodents have disappeared and their place has been taken as pests by snakes. Fields are crawling with vipers, forests and swamps with their own snakes.
Overwhelming: The sky darkens and mealstroms and tsunamis litter the seas. Rivers and lakes flood the lands around them. Storms and rain ravage the air and the forest and swamps have fused to a uniform decayed jungle. Bigger and bigger snakes seem to show themselves and attack indiscriminately. If one looks carefully one can see hte the faint shadows of a sea serpent's ridge fins.
Traits, tricks and manifestation
When he manifests his strong sign, Jormungand will walk Midgard, followed by serpents and snakes alike. He can alter his form but his forms are never exactly perfect. He might have a hint of scales beneath his skin, or vertical slit pupils, maybe even a tail or fang like teeth. Through his experience he has learnt to hide the signs well (DC 35 spot check). He tries to kill his prey by using his allied snakes to their advantage. If he knows that a person is vulnerable to poison he will let a snake slither in the night to the victim and have one bite be enough. If he knows someone can handle poison well he sends a snake to strangle. He will keep looking forhte gate to Nilfheimr to free his grandfather so he can finally bring about Ragnarök.