When a party encounters a monster, players often ask, “what do I know about this monster?”
This may be because the player has no idea what the monster may be, and wants to leverage any information their character might have. Alternatively, a player well-studied on fantasy fauna stat blocks may be screening their player knowledge from their character knowledge. I know that trolls are scared of fire, but does my character know that?
Either way, the player is seeking guidance on how to inform their character choices.
This resolves with an Intelligence check, adding a skill modifier such as Arcana, History, Nature, or Religion. But, the success of that roll and the amount of information provided is largely dependent on DM fiat. In order to apply this consistently, the DM needs guidance on what information to give out at different Difficulty Classes (DCs).
So, we’ve developed a system that you can apply to any monster. You roll an Intelligence check and you learn certain specifics about that monster based on your success. The benefits are cumulative, so if you roll a 10+, you learn a creatures Hit Dice as well as their senses.
So, how does this work? Let’s say you encounter an adult red dragon. Everyone who thinks they might know something about the monster rolls a knowledge check when they roll initiative. A character rolls a 22. They know that the adult red dragon has 19 Hit Dice (DC 5+), blindsight and darkvision (DC 10+), bite, claw, tail, and breath weapon attacks (DC 15+), and immunity to fire (DC 20+).
In easy mode, if they used their history skill, they also know that the adult red dragon has no weaknesses. That’s because the DC is 5 lower for using the appropriate skill, so they also learn the next item up in the chart.
In hard mode, they would not have been able to roll a monster knowledge check unless they used the history skill. Whether or not they must also be proficient in history to use the skill is a matter left to the DM.