Let's say we're fighting an enemy with piercing damage reduction, like the Rakshasa, Tataka with it's DR 15/good and piercing. A tail slap or other bludgeoning weapon, even if it's good, does not get through that. Let's now say we cast Thorn Body on the person attacking with natural weapons, which adds 1d6 piercing damage to the attack. Making up numbers the attack now does, say: (4d6 + 20 bludgeoning) + (1d6 piercing)
How does this interact with the monster's DR? I can see two interpretations:
- The new piercing damage gets through DR, but that's it, so the damage dealt is: (4d6 + 20 - 15) + (1d6)
- Now that the weapon does any piercing damage at all, it gets through piercing DR, so the damage dealt is: 4d6 + 20 + 1d6
I didn't see anything that directly addresses this in the rules, but obviously I could have missed it. Personally, I lean towards option 1.