Identifying a Monster in D&D 5e: More than an Intelligence Check.
Updated: Sep 28
You've probably experienced an encounter where a player asks, "Do I know anything about this monster?" Knowing what specie or type of creature you face – its hit points, traits, abilities, and attack options – is valuable in combat. But 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 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: aberrations, beasts, celestials, constructs, dragons, elementals, fey, fiends, giants, humanoids, monstrosities, oozes, plants, and undead. (MM p. 6.) There's no official rule on such links. But the idea of course is that if you have skill in an area relevant to the encountered creature type, then you get to add your Intelligence ability modifier (or some other ability modifier) to your d20 roll. The Difficulty Class, determined by the GM, will depend on the rarity of the creature type and other circumstances.
I've come up with my own table. I add the Wisdom (Survival) check to the list of checks. I also add a number of particular creature types to supplement the broad monster types of the MM. You can use it or one of the tables in the above link, or make up your own list.
Here are some options for going beyond the basic approach.
No chance of knowing. Rodney Thomson says that he would be "more inclined to allow a 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." These are good points. 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 or other check.
In-world justification for ability check. How do you determine if there is an in-world justification for a creature identification ability check? Mike Mearls provides a good start: "i like to use it for stuff you might hear about in a tavern or from veteran adventurers."
I think a character's background, race, class, languages spoken, and experience level could also, in the right situation, justify such ability check. For example, a barbarian who hails from the tundra could possibly know something about yeti. And more generally, I would be more inclined to give a higher level character a chance (due to the character's accumulated experience) via an ability check to know something about an encountered creature than a lower level character. The table here lists factors a DM can consider when determining if a character has a chance to know something about an encountered creature.
In certain circumstances, the factors in the table could lead a DM not only to allow a knowledge ability check but also to grant advantage on the check.
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? In other words, no check would even be necessary.
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 an encountered 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 the creature's 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 you ask whether your character knows anything about an encountered creature, the DM may determine that one of three situations exists.
There is no in-world justification for you to even have a chance of knowing anything about the creature encountered. No ability check is allowed.
There is an in-world justification for you to have a chance of knowing something about the creature encountered. You are permitted an ability check. In certain circumstances you may be granted advantage.
You are simply deemed to know something about the creature type encountered. No ability check is necessary.
The context-rich in-world approach in handling the identification of creatures is more fair and interesting. Success is steered away from an over-focus on Intelligence. 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.