To my knowledge Jormungand is the rarest boss in the game (which also makes his green hammer the rarest post-nerf green in the game).
Surprisingly, wiki info on him is still super scarce despite EotN being 15+ years old. So let's change that!
He's kind of like Maw the Mountain Heart in that he can spawn in several places in several maps, but only very rarely does. But unlike Maw in Dreadnought's, Jormungand has no set spawn location at all.
In all the time I've spent in EotN since it released, I've seen him exactly 3 times. All 3 times were in Bjora Marches. Unfortunately the screenshots of these spawns are on old machines I no longer have, but I'll go ahead and mark their locations on the Bjora map.
The idea here is for anyone else who encounters him to post screenshots of the location so we can have a better record of them (to then mark on wiki). I'll update the maps with spawn points as the screenshots come in (hopefully we're lucky enough).
Bonus points if any of the sightings can confirm whether or not there were other spawns of a single Frozen Elemental or Frost Wurm on the map in the instance you got Jormungand to spawn. Wiki suggests those might be mutually exclusive with Jormungand spawning. The two "recent" times I saw him, I didn't see either of the single spawns. When Jormungand spawns, he will always spawn with 2 other Frost Wurms.
Wiki also only mentions the 4 maps below, but if anyone can prove that he can spawn in Jaga Moraine or Ice Cliff Chasms, please do!
Bjora Marches
Drakkar Lake
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Norrhart Domains
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Varajar Fells