How do attribute increases affect monsters with increased skills or saves?

For example an Ancient Black Dragon has 15 (+2) Wisdom. The formula for passive perception is 10 + WISMOD + Perception_Proficiency_Bonus. For a CR21 the proficiency bonus is +7, so it should have a passive perception of 10 + 2 + 7 = 19. However, it's listed as 26.

This is 7 higher, so my suspicion is that some creatures add double proficiency, but I haven't verified that this is a hard and fast rule (is there such a thing? :p) or just a coincidence.

What I'm wondering is what to do if the Dragon has something cast on it that buffs it's wisdom enough to change the modifier. If it really is a double proficiency bonus for the Dragon it would make sense to modify its passive perception as its Wisdom changes. However if the extra 7 being equal to the proficiency bonus was a coincidence it might mean that listing a skill in a stat block totally decouples the skill value from the associated attribute.

• "something cast on it that buffs it's wisdom enough to change the modifier" - can you give an example of this? As far as I'm aware, there aren't currently any spells that increase ability scores. Aug 10, 2015 at 3:04
• You might want to rephrase the question to ask, rather, what the source of the ABD's apparent discrepancy is, then work from there. Aug 10, 2015 at 3:06
• @Miniman, I could've sworn there were at least a couple stat buffing spells but I can't find any. It does make the question much less important. Aug 10, 2015 at 7:21
• @TuggyNE, the discrepancy was simply that the ADB (and many others) have some skills that didn't follow any official formula that I new of. The MMp8 that Purple Monkey pointed out kind of hints at it, but the "usually" there troubled me. Aug 10, 2015 at 7:25

I believe this is answered on page 8 of the Monster Manual:

A skill bonus is the sum of a monster's relevant ability modifier and its proficiency bonus, which is determined by the monster's challenge rating (as shown in the Proficiency Bonus by Challenge Rating table). Other modifiers might apply. For instance, a monster might have a larger-than-expected bonus (usually double its proficiency bonus) to account for its heightened expertise.

Since a creature's proficiency bonus is based on its CR and not its ability scores, anything that changes an ability score enough to alter its modifier would also change the skills by that same amount. I.E a drop of two points to wisdom would decrease the modifier by one so the skills only get dropped one one as well.

• This is a conciser answer than mine. Good! Aug 10, 2015 at 3:52
• The "usual" is a bit annoying, but the rule seems to hold true for everything I've looked at. Aug 10, 2015 at 7:26

Analyzing the complete section of monster creation from the Dungeon Master Guide (from where the proficiency bonus/Challenge rating info is given in a table for monster creation), I would say it is a double proficiency bonus.

The table with the proficiency bonus for a given CR is a base which you are to use to create your own monsters (I assume the developers used a similar method). In step 17 of said process they mention skill bonuses, basically saying that if it makes sense that a monster is better at something than others (by having better senses overall, mastery of a skill, sheer awesomeness, etc) you can give it double proficiency bonus for a given skill.

Running the numbers as you did, it is most probably a double proficiency (10+2+7x2= 26) because it either makes sense that a dragon has better senses or it wouldn't be challenging if anybody could sneak on it. In that case, changes to its wisdom bonus should increase or decrease its passive perception by simple addition and substraction.

I would be hesitant to call it a "double-proficiency" bonus though. That implies that if the monster's proficiency bonus increases, so would this bonus. The rule seems to suggest that the monster is simply given a bonus, "usually double its proficiency." It's that usually that hangs me up. I would say that in an instance where an effect might cause a monster's proficiency bonus to increase, that effect would have no affect on this "larger-than-expected" bonus. This bonus remains static, in my opinion, unless you are causing a proficiency bonus to increase based on leveling up a monster. In that case, I would increase this bonus to correspond with the new proficiency.

• Hi, welcome to the site. I suggest you take a look at the Tour—we do things a little differently here. The site is strictly about Q&A, not discussion. The format of the site is heavily tailored for Q&A, and poor for discussion, and discussion isn’t the purpose here. Your answer here seems more like musing and personal opinion than an answer—a good addition to a discussion, but not an answer. For instance, you don’t really explain why you think the extra unaccounted for +7 should be treated as static, rather than a doubling of the proficiency bonus. And that’s important. Aug 25, 2016 at 13:50
• So basically, this answer would be far better if you could describe something in the rules, something in developer discussions or comments, something from your personal experience trying things one way or the other, that motivates treating that +7 as static rather than the proficiency added again. As it is, this answer skirts the line of not being an answer, and may be deleted as a result (you could still edit it and have it undeleted if that did happen, though). Aug 25, 2016 at 13:50