How does the poisoned condition affect the challenge rating?

Question

I am planning an encounter against different Ogres (Half-Ogre, Ogre, Ogre Battering Ram...). During the fight, the Ogres are drunk and suffer the poisoned condition (disadvantage on attack rolls and ability checks). How would the poisoned condition affect the challenge rating of this encounter? Can I just halve the challenge rating since the disadvantage on the attack rolls will around halve the expected damage?

And as a follow-up question: Is there an easy way to determine the effects of different conditions on the challenge rating? I assume the disadvantage on attack rolls is in my encounter (with the Ogres doing the attack rolls) much more relevant than it would be in encounters against a wizard where the party would need to do some saving throws.

Answer 1

You decrease the difficulty class of the encounter by one step

Circumstantial conditions do not affect the CR of the monsters, they modify the difficulty class of the encounter. This is treated on p. 85 of the DMG:

Increase the difficulty of the encounter by one step (from easy to medium, for example) if the characters have a drawback that their enemies don't. Reduce the difficulty by one step if the characters have a benefit that their enemies don't. Any additional benefit or drawback pushes the encounter one step in the appropriate direction.

The characters have a benefit their opponents do not: they are sober.

So if that encounter would have been deadly with sober ogres, you instead treat it as hard.

Because of this there is a very easy way to determine the effects of different conditions on CR: there is no effect on CR. CR is a fixed number that does not change. What you do change in encounters are the XP values for the CR based on number of monsters, and encounter difficulty, based on circumstances.

As an added note, both CRs and the rules for calculating encounter difficulty are notoriously inexact. For example an encounter with 3 or with 6 creatures of a given CR has exactly the same encounter multiplier of x2 (p. 82 DMG) and overall XP value, even though by all reason the encounter with twice the number of opponents should be twice as hard. So, don't sweat those CR calculations. Focus your precious prep time on something else instead.

Answer 2

Ogres are pretty much CR 2, regardless of the condition

Challenge rating is tricky. Albeit we have a generic table for CR calculations, the final value is mostly empirical. For instance, Ogres are CR 2 because of their one-shot damage:

the ogre is a CR 2 creature because its damage is enough to drop most 1st-level characters in a single hit

Disadvantage on attack rolls doesn't change this much, so this CR still should be 2.

Challenge rating is also deceitful. It gives you a feeling of "balanced" (hopefully not boring) encounter, while the real outcome can be very situational. In the end of the day, building good encounters requires knowledge and experience. I suggest reading The Angry GM blog as a starting point:

• Four Things You’ve Never Heard of That Make Encounters Not Suck
• How to Build F$&%ing Awesome Encounters!
• How to F$%& CR: A Practical Example of Encounter Building the Angry Way

Answer 3

Recalculate the CR with a -5 penalty to hit

A poisoned creature has disadvantage an all ability checks and attack rolls. Ability checks and their modifiers however do not factor into the CR of a Monster, so all we need to consider is what that condition does to attack rolls. Disadvantage is generally consider to be -5 penalty on any check, this can be see in the PHB rules about passive checks and is further elaborated in this answer. This means, to find out what challenge a poisoned monster poses, we can recalculate its CR, while giving it a -5 an all attack rolls. I will keep this process short, assuming you already know had to calculate CRs according to the rules in DMG.

Half-Ogre
Damage per Round: 13
To Hit: +6 -5 = +1
OCR: 1/2
Average HP: 30
Armor Class: 12
DCR: 1/8
Final CR: 1/4

Ogre
Damage per Round: 14
To Hit: +5 -5 = 0
OCR: 1/2
Average HP: 59
Armor Class: 11
DCR: 1/4
Final CR: 1/4 or 1/2 (your choice)

For the Ogre, note that the designer's gave it a higher CR than its actual calculation would wield (CR 2 instead of 1), because it can instantkill a level 1 PC, check out this answer for more info.

Ogre Battering Ram
Damage per Round: 15 * 3 = 45 (can attack thrice per round with multiattack and reaction)
To Hit: +6 -5 = +1
OCR: 5
Average HP: 76
Armor Class: 11
DCR: 1/2
Final CR: 3 (5,5 / 2 = 2,75 rounded up)

This doesn't really work for Monsters that use saves

Note that in general monsters who don't primary deal damage with attacks, but with abilities that require saving throws (e.g. Dragons with their breath weapons or many spellcasters) will not have their CRs changed by this recalculation. While fighting them most of their challenge will still be intact, even when poisoned, though if they still deal part of their damage with attacks (like again, dragons would do if their breath is on cooldown), the encounter will most likely still be easier than before.