Last time, we talked about all the nasties awaiting our brave adventurers above ground. Now I want to look at what (or who) might be lurking underground.

Giving it some thought, I want to split the Depths, as I’ll call them, into three main areas. The first area will be just below the surface, at roughly the depth of all the cellars, basements, and underground crypts in the city. In addition to these there will be a network of ancient passages and newer, rough-hewn tunnels connecting these older spaces. Whether pre-existing, formed by accident from whatever cataclysm befell the city, or formed by generations of the current inhabitants, the areas in this level will teem with a variety of challenges for our party. Some monsters can come from above, perhaps lairing in or exploring ancient cellars and caverns. But these spaces can also be the home to nocturnal or cavern-dwelling creatures (oozes, myconids, scorpions, spiders and so forth) and serve to give the party their first taste of the world that awaits them deeper underground. Throw in the occasional discovery of some construct or magical beast, leftover and trapped in a secret lab for centuries, just to add some spice and variety.

Depth-wise, this first layer isn’t going to extend much more than 80′ or so below the surface. The majority of the creatures in this area, being natural flora and fauna (albeit monstrous and horrible), should replenish themselves or be replaced by other creatures moving in. This level should never be entirely safe, unless the party takes extreme measures to eradicate dangers.

The middle layer of the Depths is going to be a combination of natural caverns and built-up areas (underground temples, small settlements, and so on). This area will be the group’s first real introduction to an alien environment, lightless and cool but teaming with denizens suited to the environs. While there are other creatures the characters will encounter, I’m making this largely the domain of the troglodytes. I don’t really see them enough in adventures except as random underground or sewer encounters, so I’m going to take this opportunity to have some fun with them and really flesh them out. I think a couple or three tribes, all fighting back and forth through the caverns and ancient chambers, will make things suitably challenging for my party. Of course there are other dangers for the party: cave fishers, stray elementals, ancient and intelligent undead…all of these and more could be waiting in the dark.

One of the things to keep in mind when running adventures underground: the environment is as much an adversary as any of the monsters. Natural caverns don’t always have level passageways (or passageways at all); there can be sudden drops or unstable areas; rocks or plants can be toxic. The darkness itself is a challenge that must be overcome. Do you carry light sources that allow you at least some limited range of sight, but pinpoint you (and your light dependency) for everyone to see? Gaining the ability to see in darkness could become a fairly important side-quest for the party. And while you don’t want travelling underground to become so onerous the party won’t do it, never let them forget they are in an alien place.

The last layer, the true Depths, is going to contain the biggest surprise for our party: another vast city, similar in size and scope to the Ruins above, even down to the architecture (excepting adaptations made for subterranean construction, of course). But while whatever befell the Ruins also happened here, it didn’t happen to as great an extent. This city is still largely intact. And occupied.

While it is tempting to trot out the drow at this point, I’m going to instead turn to another old favourite, the duergar. Perfectly suited to life in the Depths, and tenacious enough to rebuild after whatever ancient tragedy befell their city, the duergar have over time rebuilt much of what was theirs. Later I’ll determine exactly what the connection between the Ruins and the City in the Depths is or was, but for now it is enough to know the duergar blame the surface city for what happened. And their memories are long.

And because I can’t let anything be simple, I’m adding a fun little twist to the mix: intellect devourers. They are the perfect, behind-the-scenes schemers, literally living in and amongst their victims. They are particularly insidious, because it’s entirely possible that people known to the party from the very beginning, could simply be the host disguise of a devourer. Maybe even one of those main NPCs we developed earlier…

So there we go. We’ve sketched in the general outline of encounters for our party, and as you can see we have plenty to work on. Remember, though, we’re only going to start detailing the things the party is likely to encounter first. Details for the Depths will remain largely untouched for a while, as the party will tend to adventure above ground at first. But knowing what’s waiting down there allows us to start laying in hints and seeds for future adventures early. Not a lot, we don’t want to give too much away. But if the occasional troglodyte or duergar artefact surfaces, well, that’s perfectly natural. And if someone the characters have known begins acting strange, that’s certainly worth investigating.

Next time in Campaign Creation we’ll talk maps! In the meantime, what are your favourite underground encounters? Share them in comments.