D&D 5E Modifying an Aboleth: How to determine damage output and CR?

I'm running my players thru Night Below and it includes several Aboleth Savant monsters. Basically, they're aboleths with more hit points, spellcasting, wounded fury, and the out-of-combat ability to create special aboleth glyph rituals.

It occurred to me as I was attempting to determine their CR, that the damage numbers I was getting from offensive spells seemed unusually high.

How exactly does one calculate average damage from spells? Particularly where you need to make assumptions about how many targets are hit by the spell?

I've been using DMG pages 278 (Overall Damage Output) and 249 (Adjudicating Areas of Effect), which I've attached as images for clarity, along with my example spell - Vitriolic Sphere - from the Elemental Evil Player's Companion page 23.

Vitriolic Sphere deals a total of 15d4 damage per target on a failed save. It explodes in a 20-foot radius sphere, so according to DMG 249 I divide the radius by 5 to get the number of targets likely to be caught within: 4 targets.

DMG 278 makes it clear that when calculating average damage the MM/DMG doesn't factor in hit probabilities (in the sense of "multiply %chance to hit by damage to get DPR"). Hit probabilities only come into play as an adjustment of the offensive CR later on if they deviate from the expected attack/save DC values.

Ok, so average damage from a Vitriolic Sphere is 15d4 = 15 * 2.5 = 37.5 average damage

Now apply that to 4 targets. 37.5 * 4 = 150 damage. Is that right?

So a caster who was able to launch 3 Vitriolic Spheres over the course of a battle would have an offensive CR of 21 (referencing DMG page 274 Monster Stats by CR)? Of course, that's barring their attack bonus and/or save DC being higher or lower than normal for CR 21.

I feel like the target assumptions (intended for Theater-of-the-Mind) from DMG page 249 lead to inflated damage numbers for area effect spells/powers. Anyone else?

Unless I've been creating monsters very, very wrong, I'm pretty certain that the monster buildings rules tell you to assume that AoE effects hit two creatures, regardless of the actual area of the effect. Which seems a little bit strange, but I guess the idea is that damage dealt to multiple creatures is worth less than the same total damage dealt all to one creature

Quickleaf, the CR calculator is based on damage over 3 rounds. If you do Vitrolic Sphere each round that would be (150+150+150)/3 = 150 hp damage for a CR of 6, not 21.

Now a 5e aboleth also has legendary actions, so that adds 45 damage (tail swipe) for a CR of 9, just shy of the aboleth's overall CR of 10. I think you can probably add it without changing anything else.

Finally, I personally assume only 2 targets for areas of affect as I know my PCs will spread out an make it hard to get more than that in an area.

FowlJ said:

Unless I've been creating monsters very, very wrong, I'm pretty certain that the monster buildings rules tell you to assume that AoE effects hit two creatures, regardless of the actual area of the effect. Which seems a little bit strange, but I guess the idea is that damage dealt to multiple creatures is worth less than the same total damage dealt all to one creature

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I don't think the DMG explicitly states 2 targets, but the example given in the DMG (the white dragon breath weapon) only assumed 2 targets, but per the adjudicating areas of effect guidelines it should be more. I personally only use 2 targets because, as noted above, I know my PCs will make it hard to get more in the area.

dave2008 said:

Quickleaf, the CR calculator is based on damage over 3 rounds. If you do Vitrolic Sphere each round that would be (150+150+150)/3 = 150 hp damage for a CR of 6, not 21.

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Nope. You were looking at the defensive side of the equation. That's wrong. You need to look at the offensive side. According to the DMG pg. 274 (which I posted for clarity), CR 21 corresponds to 141-158 damage/round. And 150 damage for three rounds is 150 damage/round.

Finally, I personally assume only 2 targets for areas of affect as I know my PCs will spread out an make it hard to get more than that in an area.

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dave2008 said:

I don't think the DMG explicitly states 2 targets, but the example given in the DMG (the white dragon breath weapon) only assumed 2 targets, but per the adjudicating areas of effect guidelines it should be more. I personally only use 2 targets because, as noted above, I know my PCs will make it hard to get more in the area.

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Is the assumption of only 2 targets being caught in area effect abilities something that's commonly used? Besides that white dragon example, there's no mention of that in the DMG and I haven't heard other DMs who use that as a standard.

Aha!



I don't think it mentions spells in particular, but I knew I saw it somewhere.

FowlJ said:

Aha!

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I don't think it mentions spells in particular, but I knew I saw it somewhere.

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That's interesting! Definitely different than the page 249 guidelines for a cone which is to divide the size (60 ft for an adult white dragon) by 6 and then add or subtract 1d3 (or whatever seems right) depending on the situation. So the DMG page 249 (Theatre-of-the-mind) guidelines would put the lowball estimate of number of targets caught in the adult white dragon's breath at 3 (6 - 1d3), whereas the page 280 guidelines for making monsters says the number of targets caught in any dragon's breath weapon regardless of size is 2.

That's a pretty intense discrepancy.

EDIT: I fired off a Sage Advice rules query about this to Jeremy Crawford, so hopefully in the next month or so he'll address how to calculate # targets for area effects, and which guidelines should be used, and whether breath weapon is a "special case" or par for the course.

Quickleaf said:

Nope. You were looking at the defensive side of the equation. That's wrong. You need to look at the offensive side. According to the DMG pg. 274 (which I posted for clarity), CR 21 corresponds to 141-158 damage/round. And 150 damage for three rounds is 150 damage/round.

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Yep I was looking at HP, not DPR - my bad. Personally I would not add the extra 5d4 for all three rounds. Here is how I would figure the damage (using 2 targets):

Round 1: 25*2 = 50
Round 2: 25*2 + 12*2 = 74
Round 3: 25*2 + 12*2 = 74

DPR: (50+74+74)/3 = 66 or CR 10.

Now if you keep the 5e aboleth the DPR jumps up to 109 and CR 17 with its legendary actions, but if everything else is the same the total CR would only jump up to CR 11 (low defensive CR and average casting DC)

Quickleaf said:

Is the assumption of only 2 targets being caught in area effect abilities something that's commonly used? Besides that white dragon example, there's no mention of that in the DMG and I haven't heard other DMs who use that as a standard.

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I can't really say if it is common or not, I haven't specifically asked other DMs. It just works for my group. There is no reason to assume I'm going to get 4 targets when I know my group will make it hard to get even 2!

FowlJ said:

Aha!

View attachment 71530

I don't think it mentions spells in particular, but I knew I saw it somewhere.

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Any chance you could provide a pg #?