Deepwater Beacon
This underwater lighthouse stands at the edge of Umberlee's Cache, lit by a magical light which guides the merfolk patrols and keeps the sea wraiths confined within the caverns below.
The light is actually the terminus of a one-way portal from a point high above the atmosphere through which sunlight pours twenty-four hours a day. The lighthouse serves the merfolk as a tethering spot for giant seahorse steeds, as an armory, and as a storage facility. It also serves ships above by acting as a subsurface beacon.