Pretty simple idea. What if someone wants to make a weapon that WON'T work very well certain creatures? Good for evil (or even some good) overlords who don't share a creature type with the common targets of the segment of their soldiers who are issued magical weapons. Also good (but perhaps more expensive), for "reversed" Holy/Unholy/Axiomatic/Anarchic (although the ones that are designed to be teh phail against chaotic individuals would probably be a little less common).
The first question then becomes what level of countering is required to make this reasonable?
Two levels exist:
- What the GM might prefer so as to make it a trade-off (much like the Paladin CAN grab the BBEG's sword out of its sheath at his side if he rolls enough natural 20's on dexterity type checks, and go to town on the BBEG, he will just be taking a negative level for it).
- What the creator would wish, which might be more along the lines of "This should probably be worse than the average kitchen knife against the wrong sort of target."
Options I have thought of include:
- No enchantments apply. It probably doesn't even count as a magic weapon for purposes of bypassing the creatures DR.
- Enhancement bonuses apply as negatives instead of positives. Energy effects (flaming, frost, shocking, etc) lash out at the weilder any time they make an attack that succeeds as a touch attack (even if it isn't enough to actually count as a true hit).
- See Back Biter spell from Complete Arcane.
- Negative levels... maybe LOTS of negative levels.
- Simply always inflicts no damage and no effects requiring damage on the "untarget" alignment/creature type.