The Merfolk who clash with the Osprytes over Talonrook are only a small faction. Most Merfolk don’t aspire for small patches of land on which to lie. They would much prefer that the land be made sea. Some actively work (with the help of sea gods) to expand the sea into land, often beginning with the shores from which troublesome fishermen come, flooding fishing villages without warning. The Underreef is exhibit A. Dug in under Lyseria’s coastline near a town south of Norcrest, it is an elaborate front line and an ambitious staging point for the local Merfolk.
The primary purpose of the Underreef is to contain the massive shrine at A. This shrine is symbolic of the sea overwhelming land. While the shrine is connected to the ocean at high tide, at low tide it is a large, independent body of water under the land. A large altar at the bottom of the pool is used by merfolk worshipers of Dregat to pray for the expansion of the ocean in this place and to perform the required rites.
The chambers at B are also permanently flooded and are where the worshippers attending the shrine sleep (B1) and eat (B2). These worshipers might be low level (0) and classless, but some will certainly be high level illusionists, battle mages, or even enchanters.
The most forward staging point at D is rarely visited by merfolk. It is always dry and is an arduous climb on dry land for a merfolk. It is intended for when the time of invasion has come. Chambers C, H, and I are also always dry, but are fairly easy to access at high tide. These chambers are meant to be forward command stations and staging points when the water begins to rise. Chambers E and F flood at high tide and small garrisons of brawlers and low level casters move into position there at each high tide, just in case it is the moment of the invasion. During high tide, several merfolk combatants can be found in these chambers and at the changes of the tides, they will be seen moving in or out.
The chamber G is a billeting area of sorts for merfolk mounts: giant eels, giant pike, etc. It is constantly under water and filled with mounts, in preparation for the impending invasion. They are also tended to by merfolk grooms (low level or 0).
Chamber L is the current command center. It is where the merfolk in command of the Underreef meet to oversee garrisons and other activities. The commanders there have at their disposal enough merfolk to hold the Underreef against responses to the invasion until reinforcements arrive (a hundred or so), but they are mostly camped outside of the tunnels themselves.
The main entry for merfolk into the Underreef is at K. Some troops are stationed there, and in the event of action, their job would be to hold the gate until the full garrison can arrive.
Another steeper and deeper entrance at J is intended for the merfolk’s highest commanders to use to get to the front line command post at C when the invasion begins. This command post is near the two hidden hatches to the surface that are nearest the fishing town. Another passage nearby (on the surface) is primarily for troop deployment, these two will also be used by messengers. Two other hatches further away are meant for use by mounted troops, where they can egress safely and then take flanking actions.
Would you have your players storm the Underreef? How would they find out about it or find it? Would they have to deal with it after the invasion has begun, or would they get some warning? Let us know in the comments below.