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I'm working on a big creature with an AC of 22 and for flavour reasons, I'd like attacks against it to be made with disadvantage. Could you suggest how this would effect the creature's CR?

I considered halving its hit points, but as this creature will be facing 20th level characters, some may have abilities to negate the disadvantage. This would leave the creature very light on hit points. I'm wondering how to best navigate the variables to assign an effective CR modifier to the disadvantage trait.

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  • \$\begingroup\$ @as.beaulieu Please don’t post answer material using comments. See the FAQ for why your comment was removed. \$\endgroup\$ – SevenSidedDie May 10 '18 at 15:43
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According to the DMG, disadvantage on all attacks is worth about +2 to effective AC. Probably. With some caveats.

The table of Monster Features and their effect on CR lists the Faerie Dragon's Superior Invisibility as being worth +2 to AC. For reference, Superior Invisibility essentially allows the Faerie Dragon to remain invisible at all times, unless it is concentrating on a spell. Invisibility gives disadvantage to all attacks, so this is almost the same as what you're looking for. Except, of course, that it cannot cast spells while using it. Your monster presumably won't have the same restriction.

However, it's possible that this is accounted for. The majority of monsters don't use spells, and the table in the DMG is supposed to list the effect of the feature in general, not necessarily the effect of the feature on the monster it originally came from. So it's likely that +2 to effective AC is about right.

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  • \$\begingroup\$ This is is a huge help! It would actually make good story sense for the creature to have a choice of disadvantage to hit, or to be able to use its spells. Thank you! \$\endgroup\$ – Eric May 10 '18 at 11:31
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    \$\begingroup\$ Interesting - rpg.stackexchange.com/questions/14690/… suggests that advantage/disadvantage is worth between +/- 3 to 5 points, rather than 2. \$\endgroup\$ – Artur Biesiadowski May 10 '18 at 12:55
  • \$\begingroup\$ I would stick with the DMG for RAW reference. Not admitting that DMG is perfect. But only accounting the probability of the AC would ignore any spell or effect that is AOE and therefore ignore that Disadvantage on the Attack, or requires the monster to make a saving throw. So the ability to bypass that Attack Disadvantage would still be only +2 DC. \$\endgroup\$ – as.beaulieu May 10 '18 at 14:59
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I made a similar creature that always imposed Disadvantage on all attacks that it can see. More specifically, it had permanent Foresight on it. I upped the effective AC by 4 (for the purpose of determining DCR). It was a little bit on the overpowered side, so perhaps a +5 to effective AC would have been more appropriate (I did not playtest the creature again after seeing it in action once).

For your creature, since it doesn't have Advantage on Saves, just imposes disadvantage on attacks, I would give it +2 to effective AC, modeling after Superior Invisibility as a trait (DMG 280). Raising its effective HP will make it too weak as it is still vulnerable to effects that require saves. You should note, however, that Superior Invsibility is more than just imposing Disadvantage, a creature with that trait cannot be targeted by most single-target spells that require sight. Having said that, I still think it's worth a +2 to effective AC.

As always, playtest, playtest, playtest! Number crunching isn't the final step to determine CR, try to pit it against some generic PCs and see if you need to adjust.

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    \$\begingroup\$ Since you are suggesting converting this disadvantage effect to effective AC maybe referencing this answer would be a nice addition to support this solution. \$\endgroup\$ – Sdjz May 10 '18 at 10:08
  • \$\begingroup\$ The math suggest +4, but traits such as Superior Invisibility only grant +2. I'll add that in. \$\endgroup\$ – daze413 May 10 '18 at 15:28

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