Thursday, 8 March 2018

Random Monster Tables and The Mega-Dungeon

The mega-dungeon is a staple of early D&D. Not only were there tales of the semi-mythical dungeons under Castle Greyhawk, only a few partial levels of which have ever seen the light of day even now, but there were explicit instructions given for starting DMs that basically outlined the construction of such playing spaces.

The mega-dungeon is not a random artefact of early play; it is a mechanism with a specific function and that function is carried through to the DMG’s appendices A and C. Not understanding this can have unfortunate consequences for anyone trying to build non-mega-dungeons along the lines given for Dave’s Dungeons, or any other method that uses the DMG tables as-is.

The Function of the Dungeon

The obvious function of a dungeon is as a play area for the game, not much different from the board in Dungeon! So at that level its function is simply “fun”. The physical structure of a classic dungeon: rooms, corridors, heavy doors which are hard to open, darkness and so forth are all conducive to a game that centres on exploration of a hostile environment but that’s just the type of fun.

But one physical aspect of a dungeon is interesting at a more meta-level: it’s underground and self-contained. Whereas the wilderness outside (or the city, in many early cases) has the potential to allow the characters to roam at will, the dungeon is much more curtailed, giving the potential to the DM to channel the player characters. This channelling is not only in the sense of forcing parties to go down one corridor or another, but simply in entering the dungeon and returning to it.

For the mega-dungeon is a campaign setting in and of itself. PCs can enter the dungeons of Greyhawk as spotty teenagers, and continue their career there until they are ready to found a barony. The city is simply “basecamp” where gold can be converted into xp, and which exists perhaps entirely in the DM’s head.

In this context, the stairs in a mega-dungeon are vitally important for carrying out this campaign function. Look again at how Arneson stocked dungeon rooms:

Lv CR Av (Max)
1 7 (12)
2 14 (24)
3 21 (36)
4 28 (48)
5 35 (60)

Now challenge ratings for PCs are very hard to calculate because of the effects of magical equipment, but at 1st level I’d say that a PC is probably a CR of 2, averaged across the classes. So a party of 6 should be able to take on any room on the first level of such a dungeon, with only the toughest rooms posing a substantial risk.

However, the same party only one level down faces an average challenge greater than their combined strength. So, that 1st level party does not want to go down those stairs just yet. They need xp and/or magic to boost them up to a collective CR of 24 in order to be able to treat the second level the same way they did level 1. And similarly for the deeper levels.

But, if the dungeon itself (and only itself) is to be the setting, then the dungeon itself must provide that xp and/or magic. It takes the average PHB character 2026xp and 1500gp to achieve second level. For a party of 6, that’s about 12,000xp and 9,000gp in a world where a Type VI demon is worth 4128xp (officially, I make it more like 7828).

That means large levels with many rooms and opponents, not to mention treasure, which collectively adds up to the needed xp. In other words, each level has at least one specific function: to prepare the party for the next level or kill them trying. That’s sort of two functions but they’re closely related.

We can work back form this and see what it tells us about dungeon design. For one thing, if you want the dungeon to be the source of every experience level, then it has to be a mega-dungeon. By the reverse of that coin, small dungeon levels can not be tackled by parties who are not already able to handle the worst it can throw at them.

That is, small dungeon levels which increase in difficulty as outlined above, must either be entered by parties able to cope with whatever is on the deepest level, or must be left and returned to after some other adventuring has granted the needed capabilities.

Gary’s view

But the DMG wasn’t written by Dave, it was written by Gary and he didn’t provide us with the above system. The system he does give us departs from Dave’s version in several ways:

  1. The level of the dungeon has a different effect on the number of monsters appearing,
  2. The basic number appearing is set per monster.
  3. The challenge rating of each type of monster, even on it’s “own” level, varies wildly.
  4. Monsters are rated by “level” based on XP, rather than direct CR

As an example of these items, let’s look at the master table from page 174:

1 up to 16 19 20              
2-3 12 16 18 19 20          
4 5 10 16 18 19 20        
5 3 6 12 16 18 19 20      
6 2 4 6 12 16 18 19 20    
7 1 3 5 10 14 16 18 19 20  
8 1 2 4 7 10 14 16 18 19 20
9 1 2 3 5 8 12 15 17 19 20
10-11 1 2 3 4 6 9 12 16 19 20
12-13 1 2 3 4 5 7 9 12 18 20
14-15 1 2 3 4 5 6 8 11 17 20
16+ 1 2 3 4 5 6 7 10 16 20

What does a level I monster encounter look like? Well, the easiest probably-hostile item on that table is probably the 1-4 badgers or 1-4 giant fire beetles. I rate both these as a challenge of 1½ to 6, depending on the number actually rolled.

The top end is almost certainly the 5-20 giant rats which come in at a CR of 6¼ to 25. The orcs are pretty tough too; each orc is a CR of one so the orc encounter averages 10½ (the giant rats average 15⅝).

Calculating CRs is not an exact science by any means, and the simple formula on DMG p84 is a bit too simple - I count exceptional abilities as 2 HD - but in any case weaknesses are not accounted for at all. So, very low intelligence, or fear of simple things like fire do not reduce the rating. Meanwhile, clever or dumb play (on either side of the screen) will affect the difficulty of an individual encounter, or a whole campaign.

To get back to the point, lets look at what the book tells us to do on level 2 of the dungeon if a level I monster is rolled. It tells us to double the number appearing, just like Dave’s system.

What happens if a level II monster is encountered on level 3? That’s not so clear. The text seems to suggest that the number is doubled again, but an argument could be made that the increase is 3/2, or 50% and that the doubling of 1st to 2nd level is simply 2/1. This implies that a level II monster encountered on level 5 would only be increased in numbers by 5/2, or 2½ times, rather than 8x (x2 for each level, three times).

I would suggest that this second interpretation is the correct one. Not only does it mean that one doesn’t encounter 112-192 orcs roaming the 5th level of a dungeon (merely 35-60!) but it means that, when going down, the DMG works the same way as Dave’s system. The ratio of level 3 to level 2 CR points in Dave’s system is 21/14, or 1½. The ratio of level 5 to level 2 is 35/14, or 2½.

Gary finesses things slightly by stating that level IX monsters are not augmented by duplicates but by attendant monsters of a different sort, and a similar purpose is served by my suggestion of limiting numbers to the maximum value listed in the MM.

A second difference is what happens when you roll a level III monster on level 1. In the system I outlined in “Dave’s Dungeons” you simply roll the 2d6 and “buy” the appropriate number, so the ratios are reversed from the “level I monster encounter on level 3”: 1/3 instead of 3/1.

Gary simply reduces the number appearing from the DMG table by one for each level of dungeon closer to the surface than the monster’s own level rating (I-X), so the ratio of difficulty is really based on the number appearing at the “natural” level of dungeon. This is further reduced by the fact that the DMG tables generates very few encounters with large numbers of monsters. It’s all well and good to say that you reduce the number of vampires encountered on level 1 compared to level 8, but the table only generates a single vampire, so you will encounter 1 at any level from 1 to 8; the rule has no effect.

The take-away is that both Dave and Gary’s systems are geared to producing quite similar levels of difficulty per encounter. The match isn’t perfect and Gary’s seems harsher at low levels and easier at high levels, but not by much.

This means that both systems are geared towards the mega-dungeon. The step-up in difficulty from one level to the next is enough to suggest that the party is expected to have themselves increased in level since tackling the previous one. This, in fact, is pretty well a one-sentence definition of a mega-dungeon, at least from the PoV of the players.

We can get all dry and analytical with this idea and, in combination with a rule of thumb that 25% of xp comes from monsters and the rest from treasure, plus a guess that an average group will find only 80% of treasure to get this “how to build a mega-dungeon level” chart for a four-member party:

Level Needed Monsters Treasure Total
1 8104 2026 7598 9624
2 8200 2050 7688 9738
3 17100 4275 16031 20306
4 35600 8900 33375 42275
5 65000 16250 60938 77188
6 128200 32050 120188 152238
7 207800 51950 194812 246762
8 362000 90500 339375 429875
9 580000 145000 543750 688750
10 782000 195500 733125 928625
11 986000 246500 924375 1170875
12 1156000 289000 1083750 1372750
13 1386000 346500 1299375 1645875
14 1286890 321722 1206459 1528181
15 2206667 551667 2068750 2620417

“Needed” is the average amount of xp that the level needs to grant the four members of a party enough to be ready to tackle the next dungeon level. The other columns are self-explanatory and include the 25% mark up for only finding 80% of the treasure (I assume that 100% of monsters are defeated one way or another). Naturally, a 6-member party would need 50% more.

It’s interesting to note that a single +1 sword, if sold, garners 2000gp/xp, or over 20% of the needed amount to move a party of 4 up one level.

Given Gary’s xp categories, this table implies more than 100 1st level monsters, although in fact the DMG suggests 80% 1st level, 15% 2nd, and 5% 3rd so we actually get a mix of something like 90 1st, 17 2nd, and 5 or 6 3rd level monsters.

That’s a lot of monsters.

The Sandbox

Let’s say that, like me, you don’t actually like the idea of a mega-dungeon. Even then, there is still plenty to learn from all of this. The first one is that the above chart doesn’t actually have to apply to a single dungeon. If a party needs 782,000 xp to get from 10th level to 11th level, then that’s what it needs and it doesn’t matter whether it was all gained in the 10 level of The Infinite Pit™ or in a dozen adventures on the high-seas.

When setting up a sandbox campaign that doesn’t centre on a mega-dungeon, this means that the DM has to think about both how the dungeons (and I’m including things like the Hill Giant Steading here) are structured and how many there are.

It’s an easy trap to fall into to just do a single “introductory” dungeon that is a challenge for 1st level characters, only to find that the party survives it…and is still 1st level and unable to enter the nice 2nd-3rd level module you intended to play at the next game session.

In effect, the mega-dungeon has to be cut up and distributed around the initial setting and one approach is to use the given tables to construct “inverted pyramids” which are essentially mega-levels divided into areas and stacked. To take the 1st level example from above, the 90 1st level monsters, 17 2nd, and 6 3rd level monsters could be divided into:

  • A pair of 3-level dungeons each with 2 3rd level monsters, 5 2nd level monsters, and 30 1st-level monsters; in each case the monsters are found on the “appropriate” level.
  • A lair of 30 orcs with three orc-leaders and an ogre.
  • A one-eyed bugbear bandit raiding border villages.
  • Four angry mongrel men who have taken a village priest hostage.

These would all be close to, or within, the borders of the PC’s nation and no major expedition would be needed to physically reach them. Clearing those would bring in enough xp and gold to level up and try their luck in the deeper wilderness using the 2-3rd level row of the DMG encounter table.

Notice how little difference there is between 1st and 2nd level. There is a major step up, in terms of xp needed, when trying to move from 3rd level to 4th.

I would probably change the encounter table for this approach, and eliminate all the 5% chances from the left hand side at deeper levels, so we get:

1 ≤16 19 20              
2-3 ≤12 16 18 19 20          
4 ≤5 10 16 18 19 20        
5 ≤3 6 12 16 18 19 20      
6 ≤2 4 6 12 16 18 19 20    
7   ≤3 5 10 14 16 18 19 20  
8     ≤4 7 10 14 16 18 19 20
9       ≤5 8 12 15 17 19 20
10-11         ≤6 9 12 16 19 20
12-13           ≤7 9 12 18 20
14-15             ≤8 11 17 20
16+               ≤10 16 20

This would mean that a 11th level dungeon or set of dungeons, which need to provide 246,500xp in monster xp (for a four-member party), would be “stocked” with:

Monster Lv xp Approx #
V 73,950 196
VI 49,300 66
VII 36,975 19
VIII 49,300 11
IX 36,975 4
X 12,325 1

And, again, these could be divided into any number of individual adventure locations. The above result could be used as the basis for a single 7-10 level dungeon where levels 1-5 are almost all 5th level monsters with some “inspectors” from lower levels sprinkled around, with the whole lot centred on some deep chamber where something lurks.

It’s tempting to view these sorts of pyramids as video-game affairs with “boss” monsters at the end of a long trail of minions to kill, but such things are too linear for my taste, so I’d probably make the thing quite hostile to everyone else, including most of the other monsters. But I’d be more likely to split the whole thing up into different areas.

And it’s worth remembering that these numbers are all pretty loose - for example, level X monsters by definition grant 10,000 or more xp for defeating them, so with a budget of 12,325 you can only buy one and you’ll have to do something with the other 2,325xp in terms of lower level monsters. Or, you can use two and claw back the 7,675 somewhere else.

Regardless, however, of what method you use to create the dungeons - hand-placed, Dave’s point system, or Gary’s random charts, or (more common) a combination of methods, the mechanics of the books’ experience charts, rewards, and item values means that these are the sorts of numbers you will have to deal with in order to maintain character progress.

Campaign Scenarios

So, you want to plan out a campaign scenario which takes the party from 1st level up to 10th or even 15th?

Here’s the grand totals needed for that:

Level Needed Monsters Treasure %
1 8104 2026 6078  
2 16304 4076 12228 201
3 33404 8351 25053 205
4 69004 17251 51753 207
5 134004 33501 100503 194
6 262204 65551 196653 196
7 470004 117501 352503 179
8 832004 208001 624003 177
9 1412004 353001 1059003 170
10 2194004 548501 1645503 155
11 3180004 795001 2385003 145
12 4336004 1084001 3252003 136
13 5722004 1430501 4291503 132
14 7008893 1752223 5256670 122

In other words, your party of four will need about 1,412,004xp to get to level 10 (the amount needed to move up from level 9). In the process they will collect in the region of just over 1 million gp’s worth of treasure (I’ve left wastage off this table) and defeat 353,001xp worth of monsters (about 2½ solars’ worth).

So, if you’re new to DMing, maybe start smaller :)

This, in fact is where I came in on this post. I’ve been plotting such a scenario for levels 1-9 for a while and struggling with the individual parts. Once I sat down and started thinking about the numbers it became clear that I was underestimating the amount of material needed for such a task both in terms of depth (total encounters) and breadth (number of encounter areas).


  1. >>> Look again at how Arneson stocked dungeon rooms:

    Nagora, I'm a little confused. Is the "CR" table based on Arneson's per level Power Point scale given in the FFC? I guess I'm just asking how you came up with that table.

    1. No, I'm adapting the system given in DMG. Basically, 1HD = 1, each special power is another, and each extraordinary power is worth 2.

  2. >>>One quirk of the “Daves Dungeons” system is that, actually, vampires can not be encountered on level 8 despite that being their standard level.

    I'm a bit confused by this too. I guess you mean your adaptation of Arneson's method to CR and the DMG proceedures? The protection point method Arneson details in the FFC stops with 1d10 * 50 at level 7, but we could presume level 8 would be 1d10 * 60. So that would give 60 - 600 points per room, with an average of 1 weaker room in twelve being somewhere between 10 - 600,(/1d6) and 1 stronger room in 12 being somewhere 60 - 3600 (*1d6) points. Vampires are HD 8 +3 which (assuming d8) gives them a range of 4 - 67 points. Even if you bump that cost up for special abilities, there is still points aplenty for vampires, even in the weak room.

    1. sorry vamp point range should be 11 - 67, not 4

    2. You're right; I had a brain-fart and was looking at the wrong number in my spreadsheet. I've removed that aside.