Thinking more about the Coastlands, I've been turning the idea of a megadungeon over in my mind. I've built a couple of these over my gaming career, and I have a particular fondness for them. Reading the posts over at megadungeons.com has also helped get me excited to work on something big again.
For the Coastlands, though, I want something different than the typical single-site megadungeon. Instead, I want to have a complex dungeon that connects a number of different sites throughout the entire area. I'm thinking of something more akin to the Underdark as presented in the old D1 Descent into the Depths of the Earth, rather than the more localized Castle Greyhawk or Undermountain. That way, instead of trying to tie all of my ideas together into something that is coherent, I can work on whatever ideas strike me. I don't have to worry if something is completely different than everything else I've presented, because I can always add it as a new point of interest somewhere else on the map.
To start developing it, aside from looking at megadungeons.com and some commercial stuff, I have been reading some of the old notes and poking through old maps. I have quite a few things that are going to get a makeover and get developed for this. I am also making some notes about a random generator for the connecting passageways. I'd like to have something that will generate passageways on multiple levels with various kinds of intersections to keep things interesting. I'll try to flesh that out and get it posted here soon.
In the meantime, here are some of the places that have major entrances to the Dark Realm. The hex locations all refer to the map here.
01-03: A giant sinkhole here has exposed many entrances into the Dark Realm. Despite its relative isolation, the place is well-known and heavily traveled. Denizens from below regularly come to the surface from the pit, and adventurers routinely enter the depths here.
01-07: The catacombs beneath the ruined city in this hex connect to several major passages.
02-07: The Temple of Arn conceals an underwater entrance to the tunnels. The area is said to be a haven for lizardmen, frogmen, and worse.
04-02: The ruined keep has an entrance to the caverns in its lowest dungeon.
07-02 and 07-07: The dwarven mines here connect to the Dark Realm, but all entrances to the deep tunnels are heavily guarded. Casual traffic is prohibited.
08-01: The tower here has an entrance to the underrealm in its basement. The entrance tunnel is a steep stone slide.
09-08: The entrance here is a deep, steep-sided crater. Several caves in the sides have access to the deeper tunnels.
12-02: The mine here has only recently connected with the Dark Realm. Since breaking into the caverns, the mine has been plagued by all manner of monsters.