Sunken City

Below the City of Glass lies another world sealed off from the light and air. It is a place of darkness, cold water, and dwindling pockets of stale air. It is a city sunken out of sight beneath the silty waters.
Situated in a marshy lagoon, the city must constantly contend with the problem of sinking structures. The richer inhabitants of the city are sure to build their structures on the few islands scattered around the lagoon. Their buildings are not nearly as prone to the sinking dangers as those built on the sea bed. As the silt below each building slowly gives way the structure settles a little more, and the water rises a little higher. Eventually a building becomes unusable. When this happens all of its entrances are sealed off and the new section is built on top of the last. Sometimes the lower level can be used as a basement until the eternally persistent water leaks in. When this happens the basement door is bricked off and sealed up. If a building changes hands the new owners might not know where these lower levels connect to. There are endless stories of people discovering old entrances to lower levels of their own building and finding treasures below. In reality this is very rare, but the romanticism of the idea never dies out. One tale that is more likely to be true however is less glorious. In a city without suitable graveyards and with a number of their population too poor for burials far out at sea, these sealed off areas are occasionally filled with bodies of the deceased. The practice is outlawed by the city, but a building owner about to seal up a basement sometimes find themselves approached by those who wish to rid themselves of mortal remains.
Time and water endless erodes the edges of these structures. However, when a building was built with skill and good materials, entropy can be held at bay for a long time. The oldest buildings might go down several floors into the muck before the weight of the structure begins to break apart the lowest levels. Some of the ruined lower levels trap and hold large pockets of air. This can create underwater grottos inhabited by a wide array of strange creatures. Canalwise inhabitants of the city are well aware of this and some make their fortunes by breaking into a sealed off lower level then sneaking into the structure through the basement. Sometimes their work causing a building to suddenly sink due to released air pressure, which can cut off their escape route. If the basement has already been sealed off it can trap the robbers there for the rest of their short lives. These dangers are infrequent however, and there are plenty of desperate people willing to take the risk.
Few people are aware of them, but the most dangerous inhabitant of the Sunken City are the Spirit Naga. Between the bodies of entombed dead and the watery passages, these evil aquatic serpents find this to be the perfect environment. Using their magics like boring insects, they are capable of making holes in floors and walls to connect together vast complexes of connected buildings. From their underwater homes they can prowl the canals above looking for victims. Their ability to bend the minds of their prey usually makes for quick and silent predations. Rarely small groups of spirit naga work together and form a coven not unlike hags. It is well that these creatures are incapable of working together for long, for their combined power is frightening.
In addition to the spirit naga, the habit of burying the dead in the lower levels can have terrible results. When a tomb is breached either by robbers, tunneling naga, or the result of years of settling earth, the breach can unleash a horde of ravenous undead. Lacedon, aquatic ghouls, and water logged zombie corpses are the most common creatures encountered in these corrupted tombs. Few last long once released however, their hunger for living flesh draws them upwards to the canals above where they quickly attract attention and are dispatched.