So I am DMing a Shattered star run, and one of the monsters is a Fiendish Seugathi, I am wondering how the DR works for this monster.
It has a natural DR of DR 10/slashing or piercing
However the fiendish template gives DR 5/good
So would this the monster just have DR 10/slashing or piercing? Or would it have a DR 5/good and DR 10 Slashing/Piercing?
The two DRs are separate; the monster uses whichever one gives it the biggest benefit against any individual attack.
What Zhayne said.
For example, a slashing or piercing weapon would have to contend with DR 5. A good weapon would have to deal with DR 10. A good slashing weapon bypasses both. And a weapon that is neither good nor slashing nor piercing is up against DR 10--DR doesn't stack, so you use the better of the two.