Identifying a Monster: More than an Intelligence Check.
Updated: Feb 22
As a DM or player, you've probably experienced an encounter where a player asks, "Do I know anything about this monster?" This is an important question. Knowing what specie or type of creature you face (its hit points, traits, abilities, and attack options) is valuable in combat. But D&D 5e rules don't provide a straightforward answer to this question.
Basic Approach: Lore-Based Intelligence Check.
Requiring a lore-based Intelligence check (PHB p. 177) is one simple way to do this. Chris Perkins suggests this. A number of DMs have done a nice job systemizing this approach by linking particular lore-based Intelligence checks to monster types. See MM p. 6: aberrations, beasts, celestials, constructs, dragons, elementals, fey, fiends, giants, humanoids, monstrosities, oozes, plants, and undead. There is no official rule on such links, so a DM makes a judgement call.
I've come up with a table of ability checks and the monster type or other category of creature to which they relate. You can use it or one of the tables in the above link, or make up your own list.
the Basic Approach
But is a lore check all there is?
No chance of knowing. Rodney Thomson says that he would be "more inclined to allow the check if there is an in-world justification beyond 'I have a high Int.'" Mike Mearls states: "I don't allow checks for weird creatures that a PC couldn't have possibly heard about." Also: "i like to use it for stuff you might hear about in a tavern or from veteran adventurers." Unless there is an in-world justification for you to have even a chance to know something about a creature, it makes no sense for you to be deemed to have such knowledge just because you rolled a 20 on an Intelligence check.
In-world justification for ability check. I think in-world justifications for allowing an ability check to determine character knowledge of an encountered creature includes the character's background, race, class, languages spoken, and experience level. For example, a barbarian who hails from the tundra could know something about yeti. And a high level character in general knows more about the monsters of the world than a low level character. The table here lists factors a DM can consider when determining if a character has a chance to know about an encountered creature.
The factors in the table could also lead a DM to grant advantage on the ability check: "The DM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result." (PHB p. 173.)
In-world justification for presumption of knowledge. Wouldn't it make sense that in certain circumstances there would be an in-world justification for a DM to conclude that you simply have knowledge about the encountered creature? For example, I would rule that a high-level cleric or high-level fighter will simply know that a skeleton has damage vulnerability to bludgeoning damage. This approach is essentially an enhanced version of the "Background Proficiency" variant rule in DMG p. 264
Is an action necessary?
Do you get to identify the monster for free? Or is an action necessary? I think you would not have to use your action in order to identify the creature. You're not attempting to actually do something here, but rather, are simply asking the DM about your current state of knowledge. You either know something about the creature or you don't. If you know something about it, you can adjust your own approach to attacking the monster and on your turn use your free "communication" to warn others (albeit in a brief utterance) about it. And you can, of course, still use your action to take a swing with your longsword.
It's important to note that we are not talking about engaging in the act of investigation. If you didn't already know something about the creature but wanted to examine it and try to logically discern from what you observed its abilities and features, then you would have to use an action to make an Intelligence (Investigation) check. This is because of the time you would spend in taking in the visual and possibly other sensory data and in intellectually processing the data. Matthew Mercer has taken this position.
When a player asks whether his or her character knows anything about an encountered creature, one of three situations exists.
Three is no in-world justification for you to even have a chance to know anything about the creature encountered.
There is an in-world justification for you to have a chance to know something about the creature encountered. The in-world justifications include how exotic or not the creature is, and your background, class, race, and so on. The DM may also consider whether to grant advantage due to these factors.
The factors discussed in No. 2 above justify the DM in deeming you to know about the creature encountered.
The context-rich in-world approach in handling the identification of creatures is more fair and interesting. Book knowledge is not everything. A seasoned high-level fighter with a low Intelligence score will know more about the potential threats lurking in the world than the most brilliant, low-level wizard.