Shadow leviathan
The shadow leviathans are gigantic wyrms that live in Erebus. They also are said to live in the deep 'shadow' of the Shadow Realm, where even Gods may find it hard to traverse.[1][2][3] The relationship between the two is currently unknown. They were described in detail by an unknown dragonkin in The Shadow Realm. He describes the wyrms as being large enough to wrap around whole worlds and having a need to hunt any sources of light that it finds, but would quickly die if it left its natural habitat.
They were first referenced by Zaros in The Promised Gift, described as far more dangerous to hunt than a thousand shadow cacklers combined.
The Fremennik entity Jormungor is thematically described to be very similar to the shadow leviathan, described as a wyrm that wraps around the planet and having a hatred of all other living things.
During the Sixth Age, the World Guardian entered Erebus to find any sort of aid that would deal with a vengeful Jas. Prior to her arrival, a Shadow Breach emerged in the Cradle. Seren believes that Zaros had played a role in its arrival and could tell that something inside was searching for the eggs, although it is not known if it was the leviathan or something else. The World Guardian went through the breach to enter a section of Erebus. The leviathan in the area immediately sensed their arrival as they attuned themselves to the shadow anima, full of pure hatred towards them.[4] After fully attuning themselves, the World Guardian found an idol and touched it, alarming it to their location before they fled the area.
Just as Jas broke through the Needle's effects, the leviathan emerged from the Shadow Breach and fought Jas, only winning when Kerapac defied her and attacked with both the Siphon and Needle, causing her to become sufficiently distracted. The leviathan then overwhelmed Jas and dragged her back into Erebus alongside Kerapac, closing the breach in the process. This particular leviathan was considerably smaller than described, being on a similar size to Jas herself.