Piercing damage reduction with a spell that adds piercing damage to weapons

Question

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:

1. The new piercing damage gets through DR, but that's it, so the damage dealt is: (4d6 + 20 - 15) + (1d6)
2. 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.

Answer 1

It would be the first option, except that the DR is good AND piercing. Gotta be careful that you meet all the criteria for overcoming the DR, and yes, I see you have listed that the attack is already good, but it must be good aligned. Such as with align weapon. Merely being LG, NG, or CG isnt enough.

If the DR was only piercing then only the damage from the spell thorn body is piercing, it does not affect the rest of the damage during the attack. So as you said, the spell damage is full, and then you reduce the DR from the rest of the damage.