1)
Assign the creature a rarity. This is how frequently the creature can be expected to be encountered
in the region where it normally lives. The DC is 10 for very common, 15 for somewhat common, 20 for occasionally encountered, 25 for rarely encountered, and 30 (or more) for very rarely encountered.
2)
Modify this DC based on how familiar the person making the check is with this creature's home region, based on where they are from, and what they've encountered or studied in the past. This modifier can be 0 (from the same region, or have at least studied that region), +5 (the creature is from an "adjacent" region to the identifier's home region), or +10 (the creature and identifier are from regions very "remote" from each other). If a creature is particularly "famous" or "legendary", you might also modify the DC by -5 (local fame) or -10 (world wide, or even multiversal, fame).
3)
Lower the DC based on how dangerous the creature being identified is. Divide the creature's CR by 2, and subtract this from the DC. This cannot lower the DC by more than 10, regardless of the CR of the creature being identified. I think this modifier should be based on an
adult version of the creature, in the case of creatures whose CRs dramatically vary with age (like dragons).
Then we make the check, and measure relative success. If you beat the DC by:0-4 You know the creature's type, what plane it is from, and anything that is a signature ability for that creature (breathe weapon for dragons, Feed for barghests, etc). You also know if its CR is above, below, or about the same as your character level. If the CRs are different by 4 or more for the base creature, the DM should tell you that, also. If the creature is a humanoid or something that commonly takes levels in classes to gain more power, you should also know this. Of course, you won't know whether this creature has class levels.
5 to 9 You know if it has a subtype and any other resistances or immunities.
10+ You know what other abilities the creature has (basically you know everything in the basic monster's stat block).
15+ You know about
this specific creature (if applicable). For example, you might recognize "Bob the wizard-slaying dragon", who is known to make extensive use of the Antimagic Field spell, and has trained in ways to prevent other creatures from fleeing from him (has Improved Trip and the
Snatch feat).
Retry: You may make one retry after you've seen a creature use a special ability of some kind. Being bathed in acid by a black dragon's breathe has a way of jogging your memory.
Examples: 1) Bob the farmer sees a big, giant, red lizard fly overhead (a red dragon). The DC is 15 (Dragons are occasionally encountered in Bob's world. He's from Dragonlance.) - 7 (CR 15/2: adult red dragons are
really freakin' scary, and are things that tales are retold of endlessly), with a -5 to the DC for familiarity (dragons play significant roles in the history of Dragonlance, and as such are famous worldwide, however Bob doesn't live in a region that any call home),
for a total DC of 3. So Bob can take 10 and identify the thing as a red dragon, unless he is in combat with it (or more likely running away in terror from it's frightful presence). He might have heard that red dragons breathe fire.
2) Fizban the fabulous, the local hedge wizard, also sees the dragon. He is level 5, and has 8 ranks in Knowledge: Arcana, and he's studied about dragons. His Int mod is +4, so he has a +12 to ID the dragon, and no familiarity penalty. He can take 10 (beating the DC by 10) and tell you that red dragons are creatures of fire, that they have a terrible breathe weapon composed of it, but that they are also especially vulnerable to cold-based attacks.
3) His master, Archmage Yoda, would say "Oh yeah, that's old Scar. He's an arrogant bastard who loves to mix things up in melee, but has a pitiful selection of spells. He does know "Resist Elements", though. So you probably shouldn't use a Cone of Cold against him.
