(OK this looks fucking stupid but maybe it isn’t.)
So doing a procedural-generation Vornheim style map for the Underdark is fucking insane. It’s a little like wilderness (you can go anywhere potentially), a bit like a desert (not may people around, alien environment, always running out of resources) and like a dungeon (movement severely constricted locally.)
So on the macro scale you can go anywhere, I the immediate sense you can only go to a few specific places.
But you could maybe find a way anywhere, if you looked hard enough.
Blah blah blah
So the idea is of a cave-bearing substrate.
First, take a clean A4 sheet. Roll up a complex from the experimental generator. Write the complex rules in the top.
“The Torrenting Torment of He Who Seeks,
always exposes the tracks of those who passed last
but never hides the innocent or lets the guilty flee.”
Next you need a number of directions between 3 and 5. Right now it’s a d3+2, which is awkward. We will assume four directions for this map. Put little arrows or signs so that the directions cross each other, are reasonably evenly (but not exactly) spaced and are not parallel. Now our map looks like this.
Now in real use you wouldn’t see those arrows. They are just there to indicate the directions you chose.
Now we go through each direction in turn and again generate a number for each one. In this case we will postulate 3, 4, 5 and 3 again. Then we draw that many swift freehand pencil lines across the page in that way. Like this;
There are three lines in the first direction. Now we do the other directions.
So now we have a blank page divided into some semi-random semi-regular shapes. Each of these shapes is a section.
One thing we need to do is decide which direction this map is facing. I will add a compass.
Now we begin mapping caves. A cave is just a blob right now. (I may give it more complex glyphs later on.) We start the first cave anywhere on the edge of the map, but probably on the top. I will draw this one in green.
We can do this with the Cave Generator. This tells us how many exits and entries it has and where.
Now an important rule comes into play.
Every section with a cave must have one section next to it that cannot have caves. Simply, if you have a cave in a section, one section touching it must be coloured in. You cannot place caves there, you cannot pass through it. It can be touching edge to edge or at the sides.
So, depending on how big a section is, we may have multiple caves in it, or only one. While you are still in a section, all the caves in there are considered to be closely related, they can be explored in ‘real time’.
When a cave passage leave a section, it really matters how it leaves. If the passage leaves across a line, that means those caves are on the same level, at a similar height.
I’ve labelled that as 1 in the map.
If a passage leaves a section across its join, that is across the pointy bit, that means that passage is strongly vertical. It may be an easily manageable chimney or simply a passage at a strong angle but it goes up and down more than it goes across.
I have labelled that as 2
Now we continue to fill in the rest of the Torrenting Torment of He Who Seeks. Remember the rule about un-navigable spaces.
Ok I got bored doing that but you get the idea.
Obviously this looks really shit but it is meant to be done by someone really really quickly with a pencil in about 20 seconds.
Why the dark spaces?
Fuck, I can’t even remember my reasoning but it was good. I think.. it’s important that there be places you can’t go? Where caves can’t be and could never be? So you can’t just race around the map, you have to find your way a bit.
And you can’t race anyway because the caves will only generate in certain configurations.
Rivers could move through the grey spaces. If there was a river on the map you would just draw it through after the substrate was added. If a grey space pope up, the PC’s can’t follow it. If it goes over a line that’s maybe a sump, if it hits a diagonal then a waterfall?
And if they can get breath under water and anti-cold spells and underwater lights maybe then they could reach those areas?
The lines crossing diagonal joins should have arrow on them to remind the DM if they go up or down. In case the players come back.
(There was more but its late and I'm tired.)