There is no exact formula to convert 5e monster to OSR games.
You can use this:
- Divide HP by 10. This is your number of HD.
- Divide damage by half.
- Keep AC as written [if using AAC], or add +1.
- Attacks and HP are defined by HD.
- Saves as a fighter of equal HD (e.g., 9HD monster saves as a 9th-level fighter).
But it is much better to find a monster of a similar type in your "main" monster manual and change as desired.
Here is why.
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A warning
I started writing this because I was curious. While I learned a few things about how monsters changed from TSR era to 5e, I find it that it was ultimately an unproductive exercise that took me too long. In conclusion, I think you're better off just finding a similar monster. But I decided to publish my ramblings anyway, so that maybe someone can find it useful or give me a simpler method.
(BTW, please notice I used commas as decimal points due to my version of Excel, sorry about that!)
Introduction
Once upon a time, I was playing 5e and trying to convert some of my favorite monsters from the TSR era, and
I wrote this. Nowadays, I am playing OSR games exclusively - if anything, I might convert 5e monsters to OSR stats if I want to try some 5e adventure or creature.
When someone asks me how I do this, I often say the easiest method is just dividing 5e damage and HP by two. If necessary at all - I prefer to find an existing monster in the MM or the
S&W SRD, for example.
But I got curious to run some numbers (with an admittedly small sample of 5 random monsters). Here are my results.
These are from the 5e SRD.
I also tried divide HD by CR to see if there is an obvious relation. There is not (there are other ways to use CR that could be useful, but I'm not getting into that). Likewise, the amount of HP per HD is not fixed.
And these are from the OSE SRD (I took some liberties with 4+1 HD and 1-1 HD). I used the ascending options to make the comparison easier.
Here is a comparison. AC and attack bonus columns are 5e number minus OSR numbers. The other columns are 5e numbers divided by OSR numbers.
First round of conclusions
First: there is no exact formula. Result will vary widely depending on monster type. We can, at best, try some approximations - and we are doing this with a very limited sample.
Second: 5e damage is about twice as high, as predicted. HD follows a similar pattern... but not always. HP is about three times higher.
Third: AC is a bit lower in 5e (about one point), and the attack bonuses a bit higher (about two points).
So, my first guess was about right, except 5e HP is higher than expected. But there seems to be an easier way to fix that.
Just divide damage and HD by two. Keep AC (or add a point if you will), but let attack bonuses, HP and saving throws be defined by HD. A monster with 5HD saves as 5th-level Fighter, etc.
A bigger sample
I was unsatisfied with the results and decided to try a bigger sample, using 5e HP as a measure of power. This might be a better idea since HD are different in 5e (2d4 HD to 2d20 HD are different things). So I started by dividing HP by 10 to find HD.
I removed the dragons because they are just too different between B/X and 2e, to a point I don't think you could generalize "OSR" stats.
Here are some results:
As you can see, this method works reasonably well in the first group - up to 50-60 HP. AC is usually close too. From 60-100, it starts to break down - but you can can use a similar formula, maybe subtracting one HD or two and adding one or two points to AC as appropriate.
After 100 HP, 5e monsters get a huge boost in HP, but not in AC. 5e PCs at these levels are not really comparable to OSR PCs (a 14th level 5e fighter might have 110 HP or more, while in B/X it would be maybe half as much), and I think monsters are hardly comparable either. If you get this far, you might be better off reverting to a number between HD/2 and HP/10.
I think you could find a method that's a bit more precise than the ones mentioned above, taking HD AND HP into account at the same time... but, TBH, I don't think it is worth the effort, as you'll see below.
Outliers and monster types
Some monsters changed more than others. Notice that the earth elemental has the same HD in both versions, while the ghoul went from 2 to 5, and the 5e veteran has 9 HD while the B/X version has one to three!
The dragon changed a lot from B/X to 2e, for example. I used a young red dragon in my first comparison because the "standard" in the 2e Monstrous Manual is a youngling. In B/X there is only one version (10 HD), but in the 2e MM the youngling has 15 HD, while an adult has 17 HD.
But the main thing to keep in mind are monster types. Oozes lost lots of AC. Most humanoids NPCs are frail in the OSR, with one or two HD even for veteran warriors. Giants have lots of HP in 5e. And so on.
Is it worth the effort? Super-simple conversion
Given that there is no precise formula that applies equally to all monsters, as seem above, is a careful conversion worth the effort?
I think this whole exercise is useful to learn the right "ballpark" for monsters, but it might be easier AND more precise if you just found a similar monster and adapt accordingly. For example, a Nivix Cyclops (Ravnica) has 10 HD in 5e, while a usual Cyclops has 12 HD. In OSE, a Cyclops has 13 HD. So an OSE Nivix Cyclops could start with 11 HD or so, with a few special powers.
As you notice, Cyclopes were already quite though in the B/X era.
But if you don't have a similar monster (which is rare), just decide where the monster fits in the HD pecking order, and go from there.
Use your "main" monster manual for that. For example, this is adapted from
Low Fantasy Gaming Deluxe Edition, used as an example, but you could choose the 2e MM or OSE as you "main" monster book.
< 1 HD - Bat, Giant Rat, Goblin, Kobold, Man Eating Monkey, Sprite, Stirge, Xornling
1 HD - Dwarf, Elf, Human, Projectile Leech, Skeleton, Urgot, Wolf.
2 HD - Centaur, Dire Rat, Giant Ant (Worker), Giant Centipede, Giant Spider, Green Slime, Horse, Serpentman (Hraarsk), Skorn, Will o’ Wisp, Zombie.
3 HD - Boar, Dire Bat, Dire Wolf, Gargoyle, Ghoul, Giant Ant (Soldier), Giant Eagle, Harpy, Lemure, Wererat, Worm (Plague), Yellow Mould.
4 HD - Cockatrice, Claw Toad, Doppelganger, Fire Beetle, Gelatinous Cube, Giant Lizard, Griffon, Ogre, Ogre Skeleton, Shade, Tiger.
5 HD - Barrow Wight, Chuul, Giant Serpent, Giant Shark, Giant Wasp, Hammer Snail, Hell Hound, Sorcerer, Maelheim Terror, Manipede, Merrow, Minor Elemental, Minotaur.
6 HD - Giant Ant (Queen), Giant Crocodile, Giant Scorpion, Grey Ooze, Hag, Imp, Owlbear, Serpentman (Ssurlock), Werewolf, Wraith.
7 HD - Banshee, Cyclops, Flesh Golem, Manticore, Mummy, Ogre Mage, Sabretooth Tiger, Spectre, Troll.
Etc.
Notice that a Nivix Cyclops should have FEWER HD in LFG when compared to B/X, for example; OTOH, a LFG dragon would be much stronger (20 HD) than a B/X one (10 HD).
For a completely "new" monster, just decide if it is stronger or weaker than existing ones of the same type. Creating a new undead? Ask yourself, is it weaker or stronger than a ghoul? You might decide for yourself or compare it to the 5e ghoul to find the answer.
Teratogenicon revolves around types rather than specific monsters (the
RC also has monster types, but I've used the more detailed 5e version). It has a chapters on dragons, constructs, fey, etc This is a better perspective to convert monsters, IMO (although the Teratogenicon does NOT contain much about stats, being mostly a system-less book).
Thinking of monster types is good for making coherent setting too (as also exemplified in the book's appendix). I like dragons to be especially powerful in my settings, for example, so I rather use 2e dragons than B/X dragons.
Teratogenicon contains a single table extrapolating other characteristics - including AC - from HD. Attacks are just +1 per HD (a 10 HD monster attacks with +10). This is obviously a rough approximation, but it has been working well
in practice.
The book suggests using 5e conventions for HP per HD - meaning a big monster has more HP per HD (4d10 for an ogre, for example). This is a matter of taste and not strictly necessary. I happen to like big monsters having lots of HP, but not necessarily more accurate attacks. The table below uses 1d8 for HD.
Addendum: converting AD&D to Basic and vice-versa
I do not think that converting from AD&D 1e or 2e to Basic (B/X, RC, OSE, etc.) is worth the effort. The differences are so small that you are better off using the monsters as written. The
D&D Rules Cyclopedia has some conversion guidelines, but 90% is "keep is as written or use the closest analogue".
There is 1 point difference in AC that won't matter 95% of the time. 2e morale uses 2d10, but you don't need to convert - just roll 2d10. And so on.
In conclusion...
I think converting from 5e to OSR with mathematical formulas is, unfortunately, not a worthy endeavor. Just choose your "main" monster manual and fit new monsters into the existing fauna. The resulting monsters will match your system and setting much better.
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