B/X Conversions

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

• HT = HD x 5 to x 10
• Attack Skill, Parry = 10  + (Attack Bonus / 2)
• Dodge = 6 to 10, estimate
• Damage = 1d3 to 1d6 per HD or by weapon (swing = +20%)
• Special Damage = 1d3 to 1d6 per HD (or 5 character points per HD for a power)
• Ability Scores = 10 + HD (very rough; use common sense here)
• DR:
• N2AC = No armor AC
• Descending AC = N2AC minus AC
• Ascending AC = AC - N2AC
• In GURPS, plate armor is DR 8, so keep that in mind

Some of these are ranges, so use what feels right. Not every monster is the same! Also, skills for special attacks and other abilities should be close to their attack roll.

Ability scores? When in doubt, use the closest animal value, or simply use a human base 10 and modify it for HD. In most cases, you don't need this at all since you can base many things on attack rolls.

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! Monsters use these "on the spot" skill roll-and-effect powers, from spider poison and breath weapons to gaze attacks.

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.