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Converting Old-School D&D Stat Blocks to BRP

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I'm creating some rules for converting classic D&D monsters (AD&D, OD&D, B/X D&D) to BRP on the fly. Partly because I've got some old D&D modules (the Oriental Adventures series in particular) I'd like to run using BRP rules. Also, there are so many free, robust early edition D&D bestiaries out there, you'll never lack for adversaries!  

First off, here's my philosophy on creature stat blocks. I don't think they have to have the full BRP array of abilities and skills. Usually, all you need are just a few stats like 'Attack' and 'Defend'. If you need something more specific, you can generally make something up on the fly.

Let's take a classic D&D stat block (the following examples are from Swords and Wizardry which uses a single Saving Throw stat).

  • Fighter (Ftr5): HD 5(d8); AC 2[17]; Atk 1 longsword (1d8) or 2 longbow (1d6); Move 6; Save 11; AL Varies; CL/XP 5/240. Special: +1 on all saving throws except against spells. Equipment: Plate mail, shield, longsword, longbow. 25% chance that one of these items is +1.

Here are my proposed conversion metrics:

  • HP = # of Hit Dice * 4 (sentient humanoid creatures such as Orcs, Humans, or Bugbears max out at 15 hp).
  • Armor = If the creature has some sort of thematic armor (scales, tough skin, a shell or carapace, etc.), then each point of ascending AC above 9 = 1 point of BRP armor, up to a max of 8. Otherwise, the creature has no armor. (If you're using an old Basic/Expert D&D or OD&D stat block that only has descending AC, then convert it to ascending AC by using descending 19 - Descending AC score.).
  • DEX = Move + 3.
  • Atk %  = 30 + (HD * 5).
  • Defense % = Subtract any armor you've given the creature armor from the AC, and multiply the remaining AC number by 5.
  • Move = Use as is.
  • Save % = (21 - Save Score) * 5.
  • Morale  = Use as is (non-monster creatures sometimes don't have Morale).
  • AL = Use as is.
  • CL/XP = Remove or ignore the Challenge Level and XP.
  • Special = Usually you can use it as is; sometimes you'll need some manual adjustments to make those rules work.

So using the above conversion method, our 5th Level fighter converts to having the below stats:

  • Fighter (Veteran): HP 15; Armor 8; Dex 9; Atk 85% to hit, 1 longsword (1d8 dmg) or 2 longbow (1d6 dmg); Def 45%; Move 6; Save 50%; AL Varies. Special: +5% on all saving throws except against spells. Equipment: Plate mail, shield, longsword, longbow. 25% chance that one of these items is +5%.

Here's how you use each of the stat:

  • HP = Use as normal hp.
  • Armor = Use as normal armor reduction.
  • DEX = Use for initiative.
  • Atk %  = Roll for all attacks.
  • Defense % = Use for any Parry or Dodge rolls. 
  • Move = Use as is.
  • Save % = Use for any opposed rolls or to avoid the effects of a power. A successful Save roll means that the creature avoids the threat (such as a spell) or lessens its effect (perhaps taking only 1/2 damage).  
  • Morale  = Monsters' morale is rated from 2–12, the higher, the more fearless. A score of 2 means the monster won't fight and no check is needed. A score of 12 means the creature will never flee. For all other scores, the GM rolls 2d6. If the number if greater than the creature's morale rating, the creature attempts to flee. Check for each creature's morale on two occasions: (1) the first time one of their number is killed, and (2) when half of their number is killed or incapacitated.
  • AL = Use alignment to inform the narrative as to how that creature behaves.
  • Special = Use as is based on your conversion above.

Finally, here's a couple of additional examples:

D&D Stat Blocks:

  • Orc: HD 1; AC 6[13]; Atk spear (1d6) or scimitar (1d8); Move 9; Save 17; Morale 8; AL C; CL/XP 1/15; Special: none.
  • Adult Red Dragon (11HD): HD 11; HP 44; AC 2[17]; Atk 2 claws (1d8), bite (3d10); Move 9 (fly 24); Save 4; Morale 10; AL C; CL/XP 13/3900; Special: breathes fire (3/day, 90ft cone, 44 damage, save for half).

BRP Converted Stat Blocks:

  • Orc: HP 5; Armor 4; Dex 12; Atk 35% to hit, spear (1d6 dmg) or scimitar (1d8 dmg); Def 45%; Move 9; Save 20%; Morale 8; AL C; Special: none.
  • Adult Red Dragon (Ancient): HP 44; Armor 8; Dex 12 (27 if flying); Atk 85% to hit, 2 claws (1d8 dmg), bite (3d10 dmg); Def 45%; Move 9 (fly 24); Save 85%; Morale 10; AL C; Special: breathes fire (3/day, 90ft cone, 20 damage, save for half).

Lastly, there are a couple of hit point adjustments I sometimes make.

I will adjust hit points up or down as I see fit. Sometimes I'll do this on the fly at the table during the battle.

For a mob of characters, I will consider the creatures as 'Minions' with only 5 hp per creature (or perhaps only 1 hp per creature). Groups of minions can be treated as a mob where excess damage needed to kill a creature rolls over to another adjacent creature in the same mob.

So, what do you think? Any adjustments you'd recommend to the math above? Or is there any missing 'must have' BRP stat that I should always include in the conversion?

Edited by Stan Shinn
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Have you seen the "Big Damn Book of Monsters" ?

It's the AD&D MM/FF/MM2 trilogy of monster-splats, re-done as BRP monsters.

As a non-commercial fan initiative, it "flew under the radar" for years; but then The Powers That Be noticed it was flagrantly violating copyright, so it's not available for download anymore.

I suspect the right Google-fu could find a copy lurking on teh interwebz (but my casual fu did not suffice).

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C'es ne pas un .sig

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On 5/23/2023 at 4:07 PM, Stan Shinn said:

So, what do you think? Any adjustments you'd recommend to the math above? Or is there any missing 'must have' BRP stat that I should always include in the conversion?

I think you might want to be careful. While a direct formulaic approach is good for converting stats of creatures and things relative to each other, that doesn't mean that converted creatures and items will work out the same in BRP, and will match up well with your BRP PCs. Some abilities (such as fireball) are devastating in BRP while many old D&D and AD&D creatures (i.e. griffins) are weaker than their BRP counterparts. You'll probably have to adpat things a bit more flexiby to get it to work out right in standard BRP.


Now, as others have already mentioned, Classic Fantasy does a good job of adapting the old school D&D style of play into BRP.

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Chaos stalks my world, but she's a big girl and can take of herself.

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