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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 to start with. Just ignore THAC0.
  • 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.
  • A good rule of thumb is that OSR 1 HD = 1 HP, keeping in mind that the minimum is 3 or 4 HP (Cairn). I usually think of HD as equivalent to a d6, which has a mean of 3.5.
  • Hit Protection is not health. It’s the creature’s ability to avoid danger, whether through toughness, speed, or skill.
  • Strength is both health and physical power. Sometimes HP/HD from other systems is better represented in STR than in Hit Protection.
  • Willpower is tricky. It rarely comes up but when it does, it’s nice to have. High WIL is strong personality, spirit or presence.
  • 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.
  • Spells are tricky; you can give wizardy-types Spellbooks but remember, the creature might drop them when they are defeated. I like to make 1d4 dropped Spellbooks implode (Die of Fate).
  • Magical creatures can just “know” a bunch of spells. In this case make their corpse magical (and dangerous).
  • Sometimes a direct stat to stat translation isn’t an option. That’s OK! There is a way (see below).

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 OSR creature:

Foxwoman

Can take the form of a fox, a woman, or a 7’ tall fox-headed.
HD d8 HP HDx4 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.
  • 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. She probably has higher than average Hit Protection (regular humans have 3 HP). 6 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.
  • Foxes are pretty cunning, right? Probably forceful and proactive in a tough situation. 11 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 statblock:
6 HP, 12 STR, 14 DEX, 11 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.

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

Foxwoman

6 HP, 12 STR, 14 DEX, 11 WIL, teeth (d6), claws (d8+d8)

  • 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.