I must create a system, or be enslaved by another man's. I will not reason and compare: my business is to create.

- William Blake

Monday, February 27, 2023

The HD game

The idea that you can measure how challenging an encounter is to a group of PCs is an old one; in current D&D, it takes form of a "Challenge Rating" (CR) that, as far as I can tell, doesn't work too well.

I don't use CR myself; I've been taking published modules at their word ("for 5-7 3rd level characters") and even when I create my own encounters I prefer being sensible than balanced (picking a fight against a small army of orcs? Yeah, you're probably going down.). The PCs can not rely on me to make sure that I'll balance the encounter if they want to face a big dragon as 2nd-level heroes.

However, I do think having some way of measuring "challenge" would be useful, at least to be sure a module is somewhat accurate when it writes "for 5-7 3rd level characters" on the cover. It is something I've been considering for a while.

The easiest way I can think of measuring this "challenge", in Old School D&D, would be simply using HD. I'm not sure just counting HD is accurate enough for existing D&D monsters (if you want a detailed analysis, try this post by Jens or follow Delta's blog), but here is how I would do it: just count HD on both sides, and it the number is similar, both sides have a more or less equal chance of winning.

Of course there are enough variable to make exact predictions impossible. But I wish we could abstract it all to HD when necessary - for example, in mass combat.

For example, take the 9 HD elephant - I'm using that because it has no special powers (no asterisks beside the HD). According to the table I used in Teratogenicon, it would have AC 14, 41 HP and cause 11 points of damage. The actual beast has indeed AC 14 but causes an average of about 15 damage due to trampling very often. In my game, a 9 HD creature has a +9 attack bonus, but in OSE it would be a +7 - the elephant has effectively about +10, however, again due to trampling.

Anyways, it is a decent approximation for my purposes.

So, how many cavemen it would take to kill an elephant? With 2 HD, these guys are sturdier than the average bandit, for example. Let's pit 5 of them (a total of 10 HD) against an elephant and see...

They have AC 11, 9 HP and cause 5 damage on average, with +1 to hit (in my approximation, the numbers are exactly the same, except 4 damage and +2 to hit).

I'll use the Teratogenicon numbers for this fight.

All things considered (including AC an attack bonus), the five cavemen do an average of 11 points of damage per turn, while the elephant does about 14 points of damage, hitting nearly every time.

It would take 4 rounds to kill the elephant, and, conversely, the elephant can kill one caveman per turn. The elephant wins most of the times, however, since each turn one caveman dies, diminishing damage output. So, the damage is 11, 9, 7, 4, to the last caveman dealing an average of 2 damage per turn.

Now, consider morale. Losing one ally or half your allies triggers morale checks. The cavemen would probably check morale after the first attack, and then again when half the group is dead. The elephant will not check morale at all - which is why in Dark Fantasy Basic, "creatures will also test morale when put in a bad situation". Come to think of it, maybe lone creatures should check when outnumbered and hurt).

In short, the cavemen stand no chance, unless they use superior tactics (attack from afar, etc.).

The B/X results would be similar.

And what about PCs? Well. I'd like to say they could work similarly. A 9th level fighter would not gain the same benefits of a 9 HD monster, but with items, powers, feats, etc., the power level could be similar. Notice that a 9th level wizard could have a lot less HP but enough magic powers to compensate (or even overcompensate, in most D&D games I've played).

This is not how the game works, currently - but I think it could be. It would take some math and testing (especially to take monster's special powers into account), but might be an useful exercise. And maybe comparing monsters against each other is not as useful as comparing monsters - unless, again, you're doing mass combat, one of the reasons I started thinking about this.

Finally, if you're judging modules, you probably should measure encounters separately. A group of 2nd level fighters have no chance against a dozen ghouls in melée at once, but they can easily win if they fight one or two at a time.


  1. One of my favourite systems for evaluating monster potential was Don Turnbull's Monstermark system that was featured in early White Dwarf. [Actually the discussion started in Owl and Weasel.]

    It basically consisted of determining how long it would take a first level fighter armed with a longsword to kill the monster (the D rating).

    You then calculated how many 1st level fighters in platemail and shield would be killed in that time (the A rating).

    You then applied a fudge factor for any special powers the monster might have, such as poison, invisibility, petrification, invulnerability to non-magic weapons (which is not as potent as it might seem since by the time you encounter such a creature you probably already have one). The Monstermark rating was the A rating multiplied by this fudge factor. If the monster had no special powers it's M was equal to it's A.

    For example:
    Paralysis: M = 2A
    Petrification: M = 2.5A
    Poison: M = 2A

    Don then applied his calculation to the OD&D and EPT monsters to find each one's Monstermark. He then compared the Monstermark to the OD&D method of determining "Challenge Rating" which was the dungeon level where that creature might be encountered. He found that this actually resulted in a good congruence between the monsters encountered and their place on the table, which also allowed him to consider the bounds (in terms of Monstermark) of each level on the table, so new monsters could be slotted into it.

    [As an interesting aside, the old Monster/Treasure Assortments are a useful guide for how many creatures Gygax considered an underground encounter might actually be. Although Don's original work did not take this into account.]

    1. That sounds very interesting! I had never heard of this system before. Thanks for the comment!