So the damage reduction would go away, meaning any material would hurt just as bad as the special material(s) would usually?
That’s my understanding.
I don’t think that’s true. An antimagic field suppresses an Supernatural (SU) characteristics of a monster byt not any Extraordinary (EX) effects; but damage reduction is neither an SU or EX effect, it’s just part of a monster’s base stats, like its hit point total.
That’s my best guess, too.
If you had a Supernatural ability that said something like “SU: You gain DR 5/Silver” that would go away in an antimagic field but if you just have DR on your stat sheet I think it stays. (I don’t know if any 3.5 monsters have such an ability, it’s just an example).
Damage reduction that’s overcome by magical, epic, or aligned weapons is supernatural. Damage reduction that’s overcome by materials or by weapon type (piercing, slashing, or bludgeoning) is extraordinary. This is because magic and aligned weapons would lose their properties in an AMF, but materials and damage types still work fine. Some creatures have damage reduction of multiple types (for instance, a powerful devil might have DR /good and silver, or a lich has DR /bludgeoning and magical). In those cases, the magic or alignment part of the DR is suppressed in an antimagic field, but the other part remains. So Nale likely still has DR /silver but not DR /magic or /good, and so Haley can overcome it if she’s using silver arrows (which she may well be; we don’t see the arrowheads, and we know she keeps at least some on hand).
It’s worth noting here that Nale’s transformation doesn’t exactly match anything in any of the rulebooks, and so we don’t actually know what kind of DR he has, other than that Roy’s sword does not overcome it (when magic is working). Silver and good (or a combination of those) are relatively common for devils, but his might be something different.
Having looked it up, you are correct! I kinda miss this about 3.5, 5e is a lot simpler but these deep delves into edge cases were kinda fun (when they weren’t happening at the table).
Since I looked it up anyways, it looks like youw ere correct above about damage type vs alignment/magic; materials though varies. DR/Adamantine continues to function and is EX byt DR/Silver or Cold Iron is SU and does not. So if Rich is sticking to the rules that closely, Nale is taking full damage here.
Nale is getting the best the Lower Planes have to offer, so he could have 15 or even 25 DR, so that’s actually super significant.