As a free action during combat, you can choose to analyze one target (or group of targets, if they're of the same species and class), and determine their damage resistances, damage vulnerabilities, and what actions they can take.
Additionally, if the creature does not have a specific weakness, you can choose a damage type that it is neutral to and choose for that damage type to become its weakness for the rest of the combat encounter.
This can be used as many times per long rest as half your level (rounded up).
Comments