Nook
From The Fifth City Wiki
A twisted cathedral of flesh and bone
Nook is the name of a large creature that once hunted between the stars, in the High Wilderness. For unknown reasons, it came down to the neath and embedded itself under the zee, where it currently slumbers. It seems to be similar to a long worm, entirely made out of teeth. It has been infested with parasites, similar to tapeworms. Naturally, people decided to live here, as it makes the water breathable, moreso when mixed with wine.
Nook changes the people who live in it, causing them to lose inhibitions, and become more violent. The only source of light for the citizens are strange green polyps, which are fiercely fought over.