Minimal standard for OSR monster compatiblity

I’m currently umm-ing and ahh-ing as to how to approach monster conversion.

I’ve got a homebrew game system based off White Box OD&D, and am running an open table Barrowmaze campaign using monsters from Labyrinth Lord (which is the module’s system of choice) and Old School Essentials - thus there is some conversion required to bring stat elements in line with a d6-heavy game. That’s pretty straightforward. d8 becomes a d6+1, d10 a d6+2, etc.

Armour reduces damage rather than increasing difficulty to hit (single-roll combat), however, so I need to do some quick mental calculations to convert on the fly (every 2 points of difference from an unarmoured human is roughly 1 point of damage reduction).

Saves aren’t too bad, though ‘Save as Fighter’ tends to require keeping an extra reference table close by.

But I am flicking through games like Black Hack and Swords & Six-Siders which largely base all of a monster’s figures off its HD value (HD affects damage output, comparing PC and NPC HD to determine a roll’s difficulty, etc).

Is that too lightweight to be helpful to other systems?

If you were in the middle of a game and looking to convert a monster from this OSR module the group is running through (and, indeed, from the vast library of setting material out there in other game systems), what is the minimum information you need to work out a monster on the fly?


My usual conversion procedure runs like this:

  • Monsters’ HD are converted to the same number of HD for the most combat-prone class (if classes are present) or to the generic PC HD
  • Monsters’ saves are converted to the ones of the most similar class (mostly combat-prone classes, but sometimes elves or magic users) with the level equal to the HD number
  • For damage, I just use the most similar weapon, so if the damage is like a longsword in the same system, I will use the damage from a longsword in my system
  • Armor is trickier, because there are a lot of different mechanisms. I usually try to figure a conversion table in advance and reference that.

So, for me the minimal informations are HD number, damage dice and armor value.

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So would it be easier to define both armour and attacks as descriptors, rather than as dice or numbers?

e.g. HD2, Attack as Great Axe, Defence as Leather


That would be perfect, and in fact I think that some modules already do that (I don’t remember where, but somewhere I read the stats for a snake as “HD1, attack as a dagger, defence as leather”).

Skerple’s Tomb of the Serpent Kings describes monsters like that.


The minimum for me is the flavor details. I need to know what the creature is all about and I can make the numbers up from there. I think this is especially true if you’re going for broader OSR compatibility and not just D&D-based games. A short, evocative description goes a long ways to smoothing over differences between systems when making decisions on how to convert


I agree with InkquisitiveSquid here. Hit Dice, damage and so on are important but might miss the point. A few words on how strong this particular beast is meant to be goes a long way (simple phrases like “dangerous as a bear” allows for easy comparison, even if it might read a little weird.) It’s quick and allows for understanding/converting without us needing to remember the details of the system we are stealing from.

I’m not certain if this can be done on the fly though. You would have had to read the monster description at least once before or do a bit of speed-reading during the session.


The less information you put in, the more you have to work during the conversion.

I really like the idea of “descriptive stats”, but I think they should be split up a little bit like: “strong as a bear, damages as great axe and armour as leather”.

You could even get poetic with this:

The basilisk is as hard to kill as a bear, its fangs are as deadly as greatswords and its scales are hard as leather

OK, I’m not that good at poetry, but I hope you get what I mean.


The need for a concise description is an excellent point that I’d entirely overlooked. I love OSE’s succinct summaries of its monsters.

Taken from the OSE SRD, the description for a gargoyle: Magical monsters that look like hideous, horned, winged statues. Semi-intelligent and possessed of a great cunning.

Its special abilities are also nicely laid out:

  • Blend in with stone: May be overlooked or mistaken for inanimate statues.
  • Guardians: Almost always attack when approached.
  • Mundane damage immunity: Can only be harmed by magical attacks.
  • Spell immunity: Unaffected by sleep or charm spells.

Are both necessary? Can one be amalgamated into the other, perhaps with bold text to punctuate a paragraph? Or does that lose spontaneous usability?

I’d imagine that ‘poetically’ (I too fail hard at poeming) details such as general descriptions of usual numbers and treasures could also be summed up with descriptors (hordes, hunting packs, nuclear families; tat; small shiny piles, mountains of gold). Or does that start becoming too unhelpfully vague?


I think that having both makes things quicker: you see the different elements of the creature/monster easily laid out. Or maybe use the same structure as PbtA’s moves:

When you attack a gargoyle with a mundane weapon it bounces off its skin without damaging it

Tomorrow, when it’s not late night, I’ll try to rewrite the gargoyle in this style.

I think that setting up some kind of conversion table at the beginning could help. For treasure you can say something like

This module assumes that 1GP is enough to gain 1 XP


This module assumes that 1GP is enough to pay one hireling for a week

And then put treasures and prices in GPS.


2 posts were split to a new topic: Standard for OSR wealth compatibility

Just reiterating that “armour as chain” is the ideal standard. I’ve not seen “damage as greatsword” quite as much, but I might have to start using that as well! If I’ve got HD, armour-type, damage, special abilities and most importantly the flavour, I can smooth over the rest myself. I’ve never understood “saves as level X Fighter”, that’s just too much work. GLOG gives saves = 5+HD, or the 5/10/15 split for Skerples-GLOG

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I would welcome more saves being based off HD, or some element within the monster’s stat block rather than potentially a table in another section of a ruleset / another book altogether.


Yes! For me, the minimum compatibility is problem a sentence description of the monster (key words would even do the job too!).


I’d also go with sentence or some such.

When I write, it’s usually as little as Dire Ladybird (L3, ravenous) - but that might be very little for some.

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I’m even more lightweight in a lot of my dungeon notes. Stats are usually “As Goblin” or “As Ogre” (with maybe 1-2 special abilities if applicable, or an except d10 melt ray for instance). This is for max compatibility and conserving space mostly. There’s a pretty good spread of HD and damage ranges already in most Monster Manuals.


“Armor as leather” is more common because ascending vs descending AC is an eternal debate.
Hit Dice works for attack values for the same reason, and saving throws happen to fall under the same umbrella.
“Damage as dagger” is less common because you could write 1d4 and it would translate one-to-one over the much of the swath of D&D-style games.

I’d prefer this as well because it’s easy to accomodate to whatever game it’s dropped into. Everything else is flavour.

Again, I’m in agreement, because saves should be covered by the HD (which is the level/experience/hardiness of the antagonist in question).

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Saves could be as easy as: (20 - HD)

or more complex: same as above and: HD type = d12 -2, d10 -1, d8 +0, d6 +1, d4 +2

Making a 1d4 Kobold a Save: 21 (only Nat 20), while a 1d8+1 Orc is a Save: 18