From Lost Fables Wiki
While the Sea Giants and Merfolk claim that these things are a vicious predator of the depths, like a shadow of distorted water swimming silently through the depths, they have not actually ever been seen or encountered by those who walk the surface, at least in recorded history. Supposedly they are a result of ley lines which terminate in the inky depths, where the pressure is strong enough to crush all but the stygian creatures who also frequent these depths. In form, they appear to be large aquatic animals, sometimes taking the form of huge fish, other times as a Kraken, but always made of a moving area of the hyper-pressurized liquid. Their preferred hunting tactic is to bodily swim into something and allows the pressure of their own form to crush the unfortunate victim like an egg in a Rehek's palm. Their hearts are orbs of metallic hydrogen, and dissipate almost the moment the creature itself is defeated, usually through a Sea Giant using Truths.
Near-impervious to physical weapons, as strong as the whole of the weight of the ocean.
There is supposedly a specific Truth specifically to unmake them. Otherwise, using Truths of water to tear pieces of it from its whole will gradually kill the things. The whole of it will dissipate after it loses half its total mass.