…I used to hate hearing my DM tell me that phrase. What do you mean this awesome weapon I’ve worked hard for can’t do diddly and squat (and diddly left town with Bo). Oh, it used to make me so mad. I remember actually keeping weapons I’d never use if they were a +3 or better weapon just so I could take the non-proficiency penalty to use the thing so I’d do SOME damage to whatever I was fighting. As I got older, I understood why the system was in place and even came to use it myself – though I was no where near as stingy with the magical weapons as my DM (my brother) was. I recently polled my readership on what they might like to see as a topic on my blog and this very thing came up. So I figure, hell, I can get behind this..but how to write it up for GURPS? Well, that takes a few conceits that make it near impossible to use outside of anywhere except something like Dungeon Fantasy. First, let’s take a look at what “+1 or better magic weapons” might mean in GURPS as we are all aware of (and blissfully thankful for) there is no such thing as “invulnerability,” just like there is no such thing as “instant death” with no way to resist. It might suit some game systems, but not a point-buy one. Still, approximations can be made (as GURPS Powers suggests on p. 118). First, determine the most damage that can be dealt from all the sources you wish to be “invulnerable” to. For our purposes, let’s say that the most damage that can be dealt is around 6d+18 (barbarian ogre’s high ST, Weapon Master, high-quality weapons, etc.), and round that off to around 50 points of damage. To stop all of that we’d need DR 50. Let’s further assume that anyone can get a magical weapon enchanted (e.g., any weapon with any enchantment counts as a magical weapon for this purpose) or can use magical spells that cause damage. If DR 50 vs. everything is worth 250 points, then DR 50 vs. magical weapons or magic is worth 200 points. For weapons that have either +1 Puissance or +1 Accuracy, this is worth 210 points, for those with with +2, this is worth 225 points, and those with +3 this is worth 249 points.
Alternatively, using the same cost could give you Injury Tolerance (Damage Reduction 8), which would reduce the damage, but not let you ignore it. This may be the better model for a game system like GURPS just because “you hit it, it shrugs it off” gets old and boring and will lead to players deserting your game or at least no longer being interested. This also makes it so each level could be buyable as a power-up for some powerful races. “Injury Tolerance (Damage Reduction) (Not vs. magical weapons, -20%) [20/level]” might be available to delvers.
Consider also that Supernatural Durability (Magical weapons or spells) is a third option and quite cheap compared to the others. In this case however, you still take damage – it just can’t kill you.
Picking Over the Bones
Of course, all of this is assuming you’re not just saying “this monster has this trait!” and moving on. This is done often enough in Dungeon Fantasy that were I doing such a thing in my games I would ignore the point costs altogether. The whole point of creating such a framework is to enhance the flavor of combat for some monsters, not add needless calculations.