Converting Monsters

There is no perfect system for converting from other systems.
Below are some tips that might help with the process!

The Basics

Review the instructions in the SRD. A couple of things to keep in mind:

  • OSE (or B/X) has really great stat blocks that can be easily truncated for quick conversion. See this page and this PDF for more.
  • Dungeon World has some great monster “moves” that translate to Critical Damage quite nicely, so looking for an equivalent creature in that system can really help! See this example here.
  • Sometimes a direct stat to stat translation isn’t an option. That’s OK! There is a way!

Health, Armor & Abilities

  • Hit Protection is not health. It’s the creature’s ability to avoid danger, whether through toughness, speed, or skill. If the PCs will have a tough time landing a blow that actually causes damage, the creature has high HP. A good rule of thumb is to give the creature +1 HP for every HD, on top of a minimum 3 HP (the average person). I usually think of HD as equivalent to a d6, which has a mean of 3.5. Always start with a creature’s HP first, then do their STR.
  • Armor is generally easy to map; phrases like “as leather” and “as plate” are really helpful. Generally ignore THAC0 and use descending AC (7 = Leather, 5 = Chainmail, 3 = Plate mail). If only ascending AC is given you can use (12 = Leather, 14 = Chainmail, 16 = Plate mail). Whichever system you’re converting from, the Armor values range from 1-3.
  • Strength is both health and physical power. It also tracks constitution & resistance to poisons. Look at the creature’s HD and HP (even if you’ve already done so for Hit Protection). STR is the ability of a creature to survive a direct hit, not its ability to avoid danger! If a creature is difficult to kill but not because they are good at avoiding injury, give them more STR (not more HP). A good rule of thumb is to compare them to the average person (10 STR) and go up or down from there.
  • Dexterity is probably the easiest of the bunch. Start with 10 as a base and if the creature is particularly quick (sometimes called speed or SP), agile or nimble-fingered make it go up. If it is slow to respond, bulky, or clumsy, lower the number. See Saving Throws below for more tricks!
  • Willpower is tricky. It rarely comes up but when it does, it’s nice to have. High WIL is strong personality, spirit or presence. Morale (ML) can be a good guidepost for Willpower as well. Morale typically ranges between 2-12; some games use a “Morale Check” is used to determine if a monster flees (in Cairn a WIL save is used instead). The referee rolls 2d6; if the result is higher than the monster’s ML score, they flee.

The following table offers a decent guide on converting ML to WIL.

ML 4 8 10 12
WIL 6 12 15 18


  • Attack damage is pretty straightforward coming from games like OSE (or B / X); you can usually just copy them as-is. Double-check with the weapons table if unsure.
  • Multiple attacks (e.g. 2 x claw, 1 x sting) typically convert to Blast and/or the “two weapons” rule (e.g. d6+d6 is roll 2d6, keep highest).
  • When in doubt, think about how much serious damage the creature is supposed to do. Remember that instead of raising attack damage a step, think about making it enhanced in certain situations or use the Blast and “two weapons” rule.

Abilities & Magic

  • Sometimes the mechanics of the original system simply do not translate. That’s OK; try to take what’s cool about the creature and write a “version” of their ability more appropriate to Cairn.
  • If the monster has an attack that asks the PCs to attempt to dodge or save against some ability, consider making the ability an out-of-combat “trap” that the PCs can trigger. Otherwise make it succeed automatically during combat. The Warden can always telegraph the danger prior to combat to better prepare the PCs against the danger.
  • Abilities can sometimes be made into weapons, and assign a damage die, making Critical Damage reveal the damage. More often you should simply let the ability or attack happen. Combat is dangerous, and it is up to the Warden to properly telegraph danger before the fighting begins.


  • Magical creatures can just “know” a bunch of spells. In this case, make their corpses magical (and dangerous)!
  • Spells are tricky; you can give magic-users Spellbooks but remember, they might drop them when defeated.
  • I like to make 1d4 dropped Spellbooks implode (Die of Fate), just to mix it up!

Saving Throws

  • You can rely on saving throws to glean more information about a creature’s abilities, specifically their ability to avoid death by physical trauma, magic, or poison. See the OSE SRD for more details.
  • Occasionally stat blocks will use a shorthand like (E1) or (F1) (Level 1 Elf or Fighter respectively). These are really helpful for quickly identifying at the creature’s abilities.
  • Consider adding descriptive tags such as “immune to toxic gas” or “good at dodging bullets” based on the saving throws.

The following table should help determine which saving throws should result in an increase in one of the relevant ability scores.

Death or Poison STR
Wands DEX
Paralysis or Petrification WIL or STR
Breath Attacks DEX or STR
Spells, Rods or Staves WIL

Stat Block Structure

There are many ways to do this, but try to be consistent! In Cairn I write it like this: Name X HP, X Armor, X STR, X DEX, X WIL, Weapon (dX, qualities), special items

  • Engaging descriptor of appearance or demeanor
  • Quirk, tactic, or peculiarity making this NPC unique
  • Special effect or critical damage consequence

Use The Fiction

Read the original stat block and surrounding commentary, then write a few sentences about the creature. Then convert what you’ve written to the Cairn monster stat block.

Take for example this creature:


Can take the form of a fox, a woman, or a 7’ tall fox-headed.
HD 5, Speed 120’, Armor 14, Morale 11, Attack: +4 d8hp (claw, bite or choke)

  • Defense: Cannot be harmed by metal
  • Special: Can transform into a fox or a maiden with one fox leg hidden (same stats) at will

Using the example above, I can see that she:

  • Appears as a 7-foot tall with a human woman with the head of a fox.
  • Looking at the stats, it seems like she doesn’t have too high HP, and she’s quite fast.
  • Her ML is pretty high. She’s obviously quite willful.
  • Agile and lithe.
  • Attacks with deadly teeth and claws (choking her prey if possible).
  • Transforms into a fox at will.
  • Immune to metal weapons.

What can we learn from this?

  • She’s fast, and probably savvy in a fight. Starting from 3 HP, then counting 1 HP per additional HD is 8 HP.
  • I don’t think foxes have protective hides, and she’s otherwise human beside her head. No Armor.
  • She is decently strong. Normal human is 10 and she’s bigger. 12 STR.
  • I can imagine her hunting prey over the snowy tundra. She’s fast. 14 DEX.
  • Her ML is high but not the max. And foxes are pretty cunning, right? She’ll stick it out in a tough situation. 15 WIL.
  • I’d go with bite (d6) for the teeth attack, and claws (d8+d8), the same as any two-handed weapon.

In summary, that leaves us with the following opening stat block:
8 HP, 12 STR, 14 DEX, 15 WIL, teeth (d6), claws (d8+d8)

Now on to her abilities:
This is pretty straightforward. We simply read the fictional stat block we created earlier!

  • We know what she looks like, and that she can transform into a fox at will.
  • She cannot be harmed by metal; I’m taking this to mean she’s immune to metal weapons.
  • She chokes her victims.

Easy, right? Now to make it useful:

  • Appears as a 7-foot tall with a human woman with the head of a fox.
  • Transforms into a fox at will.
  • Immune to attacks from metal weapons.
  • Critical damage: victim is choked unconscious, to be fed on soon after.

And that’s it! Behold, a converted Cairn monster:


8 HP, 12 STR, 14 DEX, 15 WIL, claws or hands (d8+d8)

  • Can take the form of a fox, or a woman with a single, hidden fox led.
  • Immune to attacks from metal weapons.
  • Critical damage: Victim is choked unconscious.