What is the correct way to calculate the Knowledge DC to identify a creature?

Question

Something that has always confused me about the Knowledge skill in D&D 3.5 is that identifying a monster with advancement is more difficult only because the HD increase. The DC of the knowledge check according to the SRD is 10 + the monster's HD. By this logic two red dragons, one a wyrmling with 7 HD takes a DC 17 check to identify and the other an adult with 22 HD takes a DC 32 check to identify. It doesn't make sense that the same creature gets harder to identify when it gets older. I recall reading somewhere that you ignore advancement with monsters when determining the DC and use the base HD. If this is the case, what do you do about monsters that don't have a base HD, like a vampire for example.

My main questions here are: Does the DC for identifying a creature use the specific creature's HD or the base HD for the type of creature. If it does use the base HD then what would the DC be for a creature that does not have a base HD.

Answer 1

Sometimes the Rules Are Guidelines...

According to the Player's Handbook, "[Y]ou can use [Knowledge skills] 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" (78). And there just ain't further guidelines; I mean, there's that ominously pregnant in general squeezed in there to allow DM fudging, of course, but otherwise the DM's on his own.

...And Then There's Lore

That changes in, I think (and comments can totally correct me if I'm wrong), July 2006, when Monster Manual IV included a lore entry for each creature. In the Manual's introduction under the Lore heading, it says of the Player's Handbook's use of Knowledge skills to identify creatures

[t]hat [the creature identification rule] addresses specific creatures very well, but there’s more to be said about creatures of general types.... As a general rule of thumb, a DC 15 check or higher will reveal all of the base creature’s type and subtype traits as defined in the glossary. This often includes information about energy resistance or various immunities. For instance, a DC 15 Knowledge (arcana) check reveals that dragons have high hit points (12-sided HD), all good saves, and have darkvision out to 60 feet and low-light vision. They are immune to magic sleep effects and paralysis effects. They eat, sleep, and breathe.

Information specific to the creature, such as its type of damage resistance, spell-like abilities, or immunities come with the high DC check result. (6)

Then, without further preamble or explanation, each of these lore entries instead of being based on the creature's HD is based on the creature's Challenge Rating (a system later adopted by Pathfinder), with the DC starting at 10+CR, a change which makes sense to a degree yet leaves unaddressed corner cases like advanced and templated creatures.

I did some research and there was no Design & Development column (yes, they're really called that--O, you wacky Wizards!) about the Monster Manual IV, and Dave Noonan doesn't mention the lore entries in his articles here and here about the design and development of the Monster Manual V, so I don't know the rationale behind the change from identification based on HD to identification based on CR. (By the way, these are the last 2 Design & Development articles published, which is a shame.) (As a further aside, I was disappointed at the removal of both the Environment and Treasure lines in the later creature entries, but that's another issue.)

Bold Assumptions

The gnoll entry in Monster Manual IV (67-71) presents gnolls of CRs of 3, 4, and 6 yet provides lore results based mathematically on the CR 1 gnoll. The design intent seems to be that when an advanced or classed creature is encountered the CR of the base creature is used to gather its lore. Templates, appear--based on the templates Lolth Touched (MM4 94), God-blooded (MM5 66), and Phantom (MM5 130)--to require a Knowledge skill check with either a DC of 15 + the creature's new CR or a DC of 15 straight up to identify the template. That's just for the template, though; identifying the underlying creature is a separate Knowledge skill check.

More Lore

An effort was made on the Wizards of the Coast message boards to compose lore entries for creatures that lacked them. The results are here, although a more table-friendly format's here.

"But What About Dragons?"

Drow of the Underdark gives us the deep dragon (114-7), the first dragon I could find post-Monster Manual IV that has age categories and a lore entry. The Knowledge check DC for the deep dragon is 15 + the dragon's CR, implying that dragons of different CRs are different creatures therefore requiring separate Knowledge checks.

Answer 2

I’m not aware of any rule that addresses dragons. Strictly speaking, mechanically, a wyrmling red dragon and a great wyrm red dragon are “different Creatures” since they have different stats. A great wyrm red dragon is not, mechanically, simply an Advanced version of the wyrmling it used to be, even though this was purely a matter of modeling the complex life path of these creatures within the confines of the rule system. Barring any particular rules specifically for dragons that I am unaware of, I think this is just a unique corner-case the rules don’t handle, and rules-as-written without an exception, yes, an older dragon is more difficult to identify than it was when it was younger.

But as I say, this is purely just a unique case that the rules weren’t designed for. There are a few similar cases (barghest to greater barghest, for example), but the circumstances of those forms of metamorphosis maybe justify a higher knowledge check.

The rules also allow a DM to supply a circumstance bonus, so you could just apply a circumstance bonus equal to the difference in HD, where the “circumstance” is that this is a dragon and therefore a special case. If you really wanted to square everything with a very strict reading of the rules.

However, I’d point out that the Knowledge rules fail in a number of ways. There are no particular rules (barring those circumstance rules above, which are purely left up to the DM) for “famous” or “obscure” monsters. Low-HD ghost-like creature #134 is easier to identify (and differentiate from all the other forms of similar creatures) than the dragons of your town’s horror stories or the angels of your church’s prayers. It’s all just rather problematic.

Instead, I usually just... eyeball the DCs. Dragons are “famous” (or notorious, more accurately) in my setting, so the DCs to identify a dragon are very low, though since I remove the color-coding-for-your-convenience, the DCs to differentiate between different sorts are a bit closer to where the rules put them and for listing the particular traits of the beast, they’re actually a bit higher. On the other hand, if a player managed to open a portal to “beyond the veil” and some cosmic horror slithered out, the DC would be very high even if the thing was low-HD and was easy for them to kill. I personally usually prefer to stick to the book where I can, but knowledge is one area where I really don’t think any hard-and-fast system can beat a reasonable DM who knows the setting.

Answer 3

The Main Question

Does the DC for identifying a creature use the specific creature's HD or the base HD for the type of creature? If it does use the base HD then what would the DC be for a creature that does not have a base HD?

It seems to be HD not Base HD, per the SRD on Knowledge:

Check: 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. [emphasis added]

It would seem logical that if it was intended to read Base HD, it would. This could arguably be read as an oversight, but interpreted in a literal way, it basically rules out the second half of the question (which makes sense from a designer's standpoint.)

Greasy Mechanics

The other main point of the question seems to be a weird disparity with the difficulty of identifying higher level creatures of the same type:

By this logic two red dragons, one a wyrmling with 7 HD takes a DC 17 check to identify and the other an adult with 22 HD takes a DC 32 check to identify. It doesn't make sense that the same creature gets harder to identify when it gets older.

It is almost certainly not intended to be logical but mechanical. Players don't know everything, so it makes no real sense to keep the DCs the same as a player progresses with better bonuses.

DM: Ok, you want to identify the creature? Ok, it's a DC 15. Go ahead and roll.

Player: Should I add my +30 Intelligence bonus?

Also, note the "In many cases" and "In general" in the original Check description. These terms seem to make it pretty clear this is an optional rule (e.g. the DM can apply it how and when he wants). It seems reasonable to NOT require a check to simply identify a familiar animal. Identification is up to the DM and the rule does not seem intended for the purpose of distinguishing essentially unchanging forms of a known creature (see the last line of the Check description.)

That said, adding HD to knowledge checks (to make them harder) can have other reasoning as well:

• Early adventurers are likely unsophisticated when it comes to monsters -- A character is familiar with a wyvern (and may think its a dragon) but has no idea what a real dragon looks like. (The dragon likely has more HD than a 1st level adventurer).

• More powerful monsters are often rarer -- This goes hand in hand with the above. Powerful creatures could decimate any given area yet likely are not often a threat. Hence, they are rarer and likely unknown.

• Players may wish/need to re-identify known but obscured creatures -- If a dragon is flying at the players through the fog, they likely need a reasonable chance of failure even with bonuses. Adding HD creates that ability to fail from a mechanical perspective by counteracting bonuses, even without modifiers for visibility.

• Older creatures are not young creatures in very important, if subtle, ways -- Of course there is the "major changes" aspect to identification (a larva is not a pupa is not an adult creature) addressed earlier. But tossing out identification for a moment, knowing whether a dragon has a breath weapon or not is important, as is whether that breath weapon shoots 10' or 30' feet. And these may change significantly between ages (human babies eat vastly different food than even young children).

As for the Lore (+CR rating) vs. old-school +HD method, I would just pick one. +CR seems reasonable, as all creatures obviously have it, its compatible with Lore without changes, and it doesn't provide as much potential variation in the same creature as going by HD (thus negating some of the lack of logic in young vs. old creatures pointed out by the original question.) As for advanced and templated creatures, there are other suggestions already made on this topic in the excellent post made by @Hey I Can Chan.

Lastly, if the system seems too off, of course you can easily change DCs based on fame with a simple, consistent system, assuming you aren't playing with RAW lawyers.

Example

• Unknown: -20/No Check (Automatic Failure)
• Virtually Unknown: +10 DC
• Rare: +5 DC
• Common: No Change
• Very Common: -5 DC
• Famous/Infamous: -10 DC
• Legendary: +20/No Check (Automatic Success)