Is there a difference between DR/evil and DR/unholy?

Question

We've got a party with a Celestial Mystic (Book of Exalted Deeds), and I was reading about his level 9 ability, which grants him DR 10/Unholy. Would this be different from DR/evil, as in it would only be breached by unholy weapons, and not attacks from evil outsiders?

I'm asking this because I haven't seen any monsters or anything that have things like DR/Unholy, Holy, Axiomatic, etc. It's usually just Good/Evil/Lawful/Chaotic. Has anyone seen any material throughout the books like this? My intuition tells me it's a typo and should be DR 10/evil, but the errata doesn't say anything about this.

Answer 1

Unholy damage reduction in Book of Exalted Deeds

Book of Exalted Deeds uses both damage reduction that's overcome by evil weapons and DR that's overcome by unholy weapons. However, BE uses DR that's overcome by evil weapons frequently—over 30 times—and uses DR that's overcome by unholy weapons only four times: the supernatural ability blessing of the glittering heaven, a level 9 class feature of the prestige class celestial mystic (56); the extraordinary ability celestial transformation, a level 10 class feature of the prestige class initiate of Pistis Sophia (65); the extraordinary ability celestial ascension, a level 10 class feature of the prestige class troubadour of stars (80); and the DR 15/cold iron and unholy of the tulani, a new kind of eladrin (171—2).

BE does not feature any instance of DR that can only be overcome by a weapon that's both evil and unholy. Further, BE never explains how to overcome DR that's overcome by unholy weapons at all. Presumably the magic weapon special ability unholy (Dungeon Master's Guide 226) (+2 bonus; 0 lbs.) is supposed to overcome such DR, but even that's technically a guess as the text is silent.

That's enough for this DM to rule that DR that's overcome by unholy weapons is, in fact, DR that's overcome by evil weapons. BE's four instances of DR that's overcome by unholy weapons is not enough to convince this DM that the BE wants the DM to develop his own new and unique rules to cover for it omitting an explanation of DR that's overcome by unholy weapons… especially when perfectly serviceable rules for DR that's overcome by evil weapons already exist.

Unholy damage reduction in the core rules

I suspect those four instances of DR that's overcome by unholy weapons stem from one very particular misstatement. This requires some introduction. The Dungeon Master's Guide on Damage Reduction, in part, says that besides damage reduction that's overcome by weapons created from special materials

Other kinds of damage reduction are overcome by magic weapons (any weapon with a +1 or higher enhancement bonus, not counting the enhancement from masterwork quality), certain types of weapons (such as slashing or bludgeoning), and weapons imbued with an alignment (such as that granted by the holy special ability or the align weapon spell). (291, emphasis mine, and—as always—examples absent from the SRD)

No mention of DR that's overcome by unholy weapons. The Monster Manual on Damage Reduction, in part, says

Some monsters are vulnerable to chaotic-, evil-, good-, or lawful-aligned weapons. When a cleric casts align weapon, affected weapons might gain one or more of these properties, and certain magic weapons have these properties as well. A creature with an alignment subtype (chaotic, evil, good, or lawful) can overcome this type of damage reduction with its natural weapons and weapons it wields as if the weapons or natural weapons had an alignment (or alignments) that match the subtype(s) of the creature. (307—8, emphasis mine, and link added)

Also no mention of DR that's overcome by unholy weapons. However, the Monster Manual v.3.5 Errata on Damage Reduction offers no information on damage reduction that's overcome by evil or good weapons. Instead, the Errata, in part, says

Damage Reduction is either extraordinary (Ex) or supernatural (Su). Use the following guidelines if it is not specified.…

• DR X/chaotic, lawful, holy, or unholy should be (Su)[.] (1)

(Just to be clear, no creature in the Monster Manual actually possesses DR that's overcome by holy weapons or by unholy weapons. And, yes, much to my dismay, the premium edition of the Monster Manual (2012) incorporates this errata as-is.) Thus it appears that the Monster Manual errata accidentally conflates holy and good as well as unholy and evil. This reader suspects that the author of those four instances of DR that's overcome by unholy weapons in the BE was misled by the MM errata.

Answer 2

This is more complicated than I expected, so I'll have to break it down in to steps.

Good/Evil weapons

Strangely, I can't find the rules for good/evil weapons in the DMG (evidence for Hey I Can Chan's answer?), but it's clear that they exist. For example, the Monster Manual lists them as an example on page 307/308, there's numerous examples of DR/good or DR/evil, and most importantly, there's the Cleric spell Align Weapon that explicitly makes weapons "good, evil, lawful, or chaotic, as you choose" and not, say, holy. This implies that there is a difference between good/evil weapons and holy/unholy weapons.

(Un)Holy weapons

Fortunately, holy and unholy weapons have rules (DMG pages 225/6). In particular, this is the relevant part for unholy weapons:

An unholy weapon is imbued with unholy power. This power makes the weapon evil-aligned and thus bypasses the corresponding damage reduction. It deals an extra 2d6 points of damage against all of good alignment.

From this, it is clear that (un)holy weapons do an extra bit of damage on top of bypassing good/evil DR. Because we don't have rules for good/evil weapons, this appears to be a unique (un)holy weapons property.

(Un)Holy damage?

This is where we run in to trouble. As best as I can tell, the DMG does not tell us what type of bonus damage is done by (un)holy weapons. It's possible that the bonus damage in question is what DR/(un)holy is intended to resist, but this doesn't actually give us rules for it. This answer suggests that we don't need to know what type of damage it is because the game has defaults for these things.

Fortunately, we can still make (un)holy damage work without (un)holy weapons. BoED and BoVD both clearly treat (un)holy damage as proper damage types and, at least internally, that ought to work. In fact, you can even find an example of holy damage on page 259 of the DMG2 (Holy Surge) and another a few pages later (Unholy Surge). Is it silly that (un)holy swords don't do (un)holy damage? Yes, but that doesn't stop us from using (un)holy damage RAW.

Your options

I see four choices.

1. Wait for a better answer.
2. Play (un)holy damage as BoED and BoVD have them. Even if they don't have rules, you can still use whatever it says is (un)holy as being resisted by DR/(un)holy.
3. Houserule it. Maybe that extra damage from (un)holy weapons actually does has its own (un)holy damage type.
4. Treat it as an error as Hey I Can Chan suggests.

Answer 3

As a DM, I read the decision to print DR/unholy as a signal that DR/evil (or DR/evil and piercing, or DR/evil and cold iron) wasn't enough, and that it's intended that specifically unholy magic weapons should be needed to deal full damage, to give them worth relative to regular weapons that have align weapon spells cast on them, and to protect better against the attacks (manufactured or otherwise) of fiends (outsiders with the evil subtype), which overcome DR/evil naturally (DMG 308).