What should be the DC to identify monster vulnerabilities?

Question

What should be the DCs for checks revealing monster vulnerabilities, damage reductions, and other qualities of the like? Knowing such facts about a monster can dramatically change the dynamic of a battle, generally in favour of the players. The PHB says that an easy question should have a DC of 10, a medium one of 15, and a hard one of 20-30. What goes in each category, though, is less clear. So how decide?

A few examples from different monsters would be nice, such as skeletons and zombies, along with other, rarer vulnerabilities for the comparison.

(Note: I know of the guideline to use a monster's HD and add ten. But as this would mean identifying damage reductions for large zombies would be harder than for small zombies, I don't think it applies in this case.)

Answer 1

The SRD contains the following, possibly abridged, text for the Knowledge skill.

Check

Answering a question within your field of study has a DC of 10 (for really easy questions), 15 (for basic questions), or 20 to 30 (for really tough questions).

In many cases, you can use this skill to identify monsters and their special powers or vulnerabilities. In general, the DC of such a check equals 10 + the monster’s HD. A successful check allows you to remember a bit of useful information about that monster.

For every 5 points by which your check result exceeds the DC, you recall another piece of useful information.

If you were attempting to answer a question about a creature as written in the monster manual, about the basic version of a creature, the check would unambiguously be as written. For example, knowing that most Dijin can't grant wishes would be DC 17.

For variants, the DC is probably based on the lowest HD which the variant applies do. So knowing that 1% of Dijin are "noble" and can grant wishes would be DC 20.

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The problem becomes more ambiguous regarding questions related to Templates, like the Skeleton or more problematically the Vampire. The idea that a giant skeleton or 20th level vampire would somehow require additional knowledge check is simply absurd, but a DC 11 knowledge check would imply that knowing the true way to kill a vampire is "really easy" -- and that it's easier to answer questions about the vampire itself than its 4 HD spawn.

For templates, or creatures which the GM judges a DC of HD +10 to be too low, the rules fairly clearly imply that the GM should defer to the more general knowledge rules. For reference sake, I believe the following DC's may be appropriate:

• That a skeleton is a dead human raised through magic may be DC 10; that they are resistant to non-bludgeoning attacks could be DC 15.

• That a vampire sucks blood and is fairly scary looking could be DC 10. That they are undead creatures which can spawn minions could be DC 14 or 15. Knowing a vampire's weaknesses might be DC 20, while knowing the true way to kill one might be higher than that, at 25 or even 30.

Answer 2

The DC I use in my games is 10 + the CR of the monster in related knowledges; for every 5 above the DC you gain one additional detail of information. The list of choices is:

Defenses: This includes all special defenses such as DR, Resistances, and Spell Resistance. This may also include any spell-like abilities that are protective spells.

Offensive capability: This lists all the attacks the monster has including special attacks and spell-like abilities that are offensive.

"How hard is it to hit?" "What are its abilities?" "How many HD does it have?" and "How fast does it move?" are self-explanatory.

There is also "What tactics does this creature usually employ?" and "Does it have any weaknesses?"

Answer 3

Besides the rules considerations offered by the other answers (a crucial aspect, of course), I think it is important to consider your world and story as well. Unless you're playing the most generic D&D in which rules of thumb do work, your campaign (story, setting, world) will have its own peculiarities that strongly influence the availability/accessibility of information about monsters.

For example, in a land where necromancy and the undead are practically unknown, because all the practicioners of the dark arts were eradicated a thousand years ago by an order of devout angel-worshippers, the DC to identify/learn the traits of a simple skeleton would probably way higher than what a general rule in a generic setting would require. There hasn't been anyone to pass even the most basic information on. There are no books, no records, no experienced undead-hunters around. So, when they first turn up in a remote forest, where the party is guarding a lone noble out hunting game, their chance of knowing anything about skeletons is minimal. DC 30-40, I'd say.

On the other hand, even though the generic DC for a bullette (CR7) would be around 17, the same guards could gain info by rolling for a DC of 5 (yes, five) in this example setting - because just last month a scholar (a powerful wizard, a brother of the noble) brought a number of live, shackled and dominated bullettes into the noble's court, where he conducted experiments on them, and has shared every bit of his findings with the guard, expecting an enemy warlord to try and use bullettes to attack the noble's domain... So the only question is how much attention each and every PC paid to the presentations of the wizard.

So, I'd recommend using a generic rule only when you don't have anything peculiar in mind for a given monster in your setting: they're just fillers, or their ecology very closely matches that of the generic D&D setting. However, if you have a particular story for a given species with which your PCs may have come into contact, let the story and the world – and, of course, the PCs' background(s) – dictate the DC, and allow the DC to be wildly different than what the generic rule of the RAW would suggest/set.

(Note that the same works well, imo, in the case of practically any knowledge skill.)