B/X Conversions

Use a standard B/X bestiary (like the excellent Basic Fantasy) and do a rough conversion:

• HP
• HD x 5  = average or weak (min 10 for 1 HD)
• Or, use "rolled HD" at 1d6+2 per
• Skills
• Attack Skill = 11  + Attack Bonus or HD
• Parry = 11  + Attack Bonus or HD
• Dodge = 6 to 10, estimate
• Be wary of values of 11-15+ as they slow down play.
• Damage = 1d3 per HD (min 1d6)
• Swing damage = +20%
• Special Powers = 5 character points per HD (min 10)
• Ability Scores = 10 + HD (very rough; use common sense here)
• ST = HD x 5 (min 10)
• When in doubt, pick a similar creature from GURPS Basic (B455) and use those.
• 10 is an average human.
• DR:
• BAC = Base, no armor AC
• In Basic Fantasy, this is 11
• Descending AC = BAC minus AC
• Ascending AC = AC - BAC
• In GURPS, plate armor is DR 8, so keep that in mind

Remember, these guidelines are not rigid rules. They're ranges that allow for customization and adaptation to different game systems. Each monster is unique, so use what feels right. Also, skills for special attacks and other abilities should be close to their attack roll.

Note that using 5 x HD for HP comes close to the GURPS HP values. You can put a minimum of 10 HP if you don't want your orcs to be 5 HP pushovers. The large red dragon in Dungeon Fantasy has 50 HP, and in Basic Fantasy, they range from 7 to 13 HD, with an average of 10 HD, which gives them 50 HP at the standard calculation.

You can "roll" HP like you do hit points in BFRPG! Roll 1d6+2 per HD for HP. That dragon would have 10d6+20 HP, or an average of 55 HP, doing it this way. Note: a 1 HD creature will only have 1d6+2 HP, like orcs, which is less than the standard human 10 HP, but you may want to leave them weaker to fit better with the genre conventions.

Attack skills can be calculated from the attack bonus (if you have it), or you can just use HD for the attack bonus number.

Using the closest animal value as a reference is best for ability scores. Alternatively, you can start with a human base of 10 and modify it based on the monster's HD. In most cases, you won't need to assign ability scores directly, as many things can be based on attack rolls. The table of creatures in GURPS Basic (Campaigns, B455) should be a good guide. If something seems as strong as a horse, use that.

If a medusa has a "turn to stone" attack, give it a skill roll based on the attack skill, and you are done. There is no need to design the power unless a character wants it! All special attacks and abilities should use a simple "skill roll" mechanic, like in GURPS Ultra Lite. Do a resisted roll if characters try to resist, such as a spider having an entangle with webs attack and a poison attack rated at a 12-. These damages should be based on special attack damage, as above, or inflict the appropriate condition.

Magic items? Most are used as-is, and magic weapons and armor can apply their bonus to damage, hit, or DR as needed. If a magic sword does an extra 1d6 fire damage, use it like that. If it is broken, fix it.

Saves? A resistance roll in GURPS.

Spells? I stick to the ones in Dungeon Fantasy and use these casting and effect rules. If one comes up in a module, I use the closest one. Magic is an easy system to swap out in any OSR game.