I'm working on a reaction which allows a creature to raise its AC in response to being hit by an attack. This reaction will be effective against any single attack that is resolved against the creature's AC.

The creature has the option of using a legendary action to recharge this reaction each turn, at the cost of making a legendary attack, so I'm trying to decide whether the damage prevented (from the reaction) or the damaged caused (from the attack) is more significant in order to decide which to include when calculating the creature's CR.

The legendary attack is a melee weapon attack that does 20 damage to a single character on a successful hit vs AC (+14 to hit). Would you use this when calculating the monster's CR, or would you choose to add to the creature's defensive effective CR in some way to simulate the damage prevented instead? This monster will face 20th level characters.


Generally speaking, the easiest way to find out is to calculate the CR twice; once with the reaction attack and once with the defensive ability, and then use the higher of the two as the final CR.

This'll probably also give you an idea of which of the two abilities is likely better. (Although having the choice is always a nice bonus)


As per the DMG pg 278:

If a monster's damage output varies from round to round, calculate its damage output each round for the first three rounds of combat, and take the average.

You'll need to essentially simulate 3 rounds of combat, then calculate the damage and defense accordingly.

Obviously this can be fairly difficult, so rather than try to manage the "what-ifs" I would use the personality of the creature to derive the number of times each ability is used. If it is more aggressive than simulate 2 attacks in your overall damage output calculations, or only 1 if it is a more defensive creature.

Then depending on the amount of AC the creature gains when it uses its other ability, you can adjust the creatures defense CR accordingly. There will be some nuance to this as well, but a good starting point would be taking the Hobgoblin Warlord's "Parry" ability, which raises the effective AC by 1, and scaling it from there.


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