This question has bought up some interesting discussion around (un)holy damage. So far, we appear to have established the following:
- There does not appear to be any link between (un)holy damage and (un)holy weapons.
- There does not seem to be any rules for (un)holy damage. However, I couldn't find any rules for good/evil damage either.
- It may be an error, where every instance of holy damage should be good damage and unholy damage should be evil damage.
- At minimum, both damage types are known to appear several times in the Book of Exalted Deeds and the Book of Vile Darkness, but they also both appear once in the DMG2's (Un)Holy Strike and the errata for the first Monster Manual.
In the interest of furthering the debate on whether or not this damage type is an error, I'd like to ask: what sources mention (un)holy damage and/or its DR? In particular, I'm interested in seeing if there are more cases outside of the BoED/BoVD, as they would be evidence against these damage types being erroneous and a potential place to look for rules about these damage types. I'd also like to see if they (or better yet, rules for them) appear anywhere in 3e.