Your studies of beastiaries, religion, history, arcana, and folklore have taught you the strengths and weaknesses of a variety of monsters.
When you encounter a monster, you may make the appropriate check based on monster type to recall information about the creature's strengths and weaknesses. To attempt a check, you must have proficiency in the corresponding skill.
Arcana - Elementals, Dragons, Aberrations, Fey, and Celestials.
Nature - Beasts, Monstrosities, Oozes, and Plants.
History - Giants and Humanoids.
Religion - Undead
Identifying and sizing up a monster outside of combat requires that you observe it undetected for three minutes from a distance of no more than 100 feet. In combat, sizing up a monster requires that you be within 30 feet. If the monster has not taken its first turn, sizing it up requires a full action. If the monster has taken at least one turn, sizing it up only requires a bonus action.
DC 5 (Very Easy) - You know the damage type and number of the monster's basic attacks.
DC 10 (Easy) - You know the damage type and number of the monster's special attacks as well as any special tactics.
DC 15 (Medium) - You know the monster's resistances and immunities.
DC 20 (Hard) - You know the monster's vulnerabilities.
DC 25 (Very Hard) - If the monster has any legendary abilities, you know one of them.
Because this ability draws on the wisdom of past monster hunters and adventures, it cannot be used on unique monsters. Because intelligent humanoids are unpredictable, checks against Dragonborn, Dwarves, Elves, Gnomes, Half-Elves, Halflings, Half-orcs, Humans, and Tieflings have disadvantage and can only reveal resistances, immunities, and vulnerabilities, if any.
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