WARNING: Math ahead
This article shows the process I went through in an attempt to “reverse engineer” the monster and come up with the challenge rating (CR) of 13 given in the book.
Quite a bit of criticism has been leveled at 5th edition’s CR system. Some of this criticism may be valid, however oftentimes these complaints reveal more ignorance on the part of the person complaining than faults of the system. One of the assumptions many/most people who level these complaints seem to make is that the guidelines for determining a monster’s CR given in the Dungeon Master’s Guide represent the entirety of the parameters considered by Wizards of the Coast when determining the CR of a monster. I think this assumption is likely false. First, I’m not sure that Wizards would show us all the secrets of “how the sausage is made”, and second, I think there is a good deal of “art” in the determination of a monster’s CR. Which is to say that it is more than just a math equation and there are numerous judgment calls that are made. The more complex a monster is, and, likely, the higher its CR, the more of these judgment calls come into play.
I learned a valuable lesson when I did a series of episodes on encounter building on my podcast (episodes 136, 137, 138, and 139). When I attempt to reverse-engineer a monster, and I get a CR that is different from what’s in the book, instead of assuming Wizards "got it wrong" or that the CR system "doesn't make sense", I first check my math. If my math is right, I then try various permutations of the calculations in an attempt to arrive at the same CR given in the book.
This may seem an odd thing to do, but I have almost always been able to find a way to reach the same CR Wizards did, or at the very least to come close. As I said, I think there’s some art to this, so coming up with a CR 1 or even 2 off from what's in the book isn't a big deal, in my opinion. Surely the folks at Wizards are privy to some guidelines and design philosophies (or just the benefit of experience) that aren't detailed in the DMG.
My assumption is that if the CR I come up with for a monster is different from what's given in the book, it's likely either due to a mistake on my part, or it's due to some judgment call or design element I'm not aware of. To assume, instead, that it's due to a flaw in the system, or that the designers don’t know what they're doing is pretentious and arrogant.
This exercise of reverse engineering a monster trying to get to the CR listed in the book has taught me quite a bit about how Wizards designs monsters and determine CRs, and how different abilities stack up in their eyes. I'm personally distrustful of monsters designed by someone who claims the CRs in the game don't make any sense or don't have any inherent logic or system to them. I doubt such a person has gained the insight needed, or has the humility required, to truly learn the design of the system and gain the level of proficiency needed to create a properly balanced monster.
Abbreviations Used in this Article:
AC Armor Class
CR Challenge Rating
DCR Defensive Challenge Rating
DPR Damage per Round
EAC Effective Armor Class
HP Hit Points
OCR Offensive Challenge Rating
The Defensive CR (DCR) of this monster is pretty straightforward. The neothelid has 325 hp, which gives us an initial DCR of 17. It has an AC of 16. However it's effective AC (EAC) would be 20: +2 EAC for having three good saving throws, and +2 EAC for having magic resistance.
(It doesn't matter in this example, but in another monster, I think you could argue not giving it the +2 EAC for the saving throws, because one of those save bonuses is only +1, because the save proficiency is canceling out its intelligence penalty. Good saving throws only affect EAC if there are three or more. But it doesn't matter in this case as the DCR will be the same either way.)
The expected AC for a DCR of 17 is 19, so having an EAC of 20 doesn't have any impact on the DCR, because it only impacts the DCR if it is 2 or more higher than the expected AC. So our final DCR is 17.
The offensive CR (OCR) is where it gets a little tricky. Unfortunately, there are two slightly different ways you could calculate the damage output, and I think both are equally "right" as far as that goes. It comes down to how a DM would run the monster in combat, so this is an instance where the "art" of the design comes in.
This monster has three ways of doing damage. First, a tentacle attack, which is super straightforward. It does 33 damage (21 bludgeoning and 13 psychic).
It also has a breath weapon attack. The guidelines state to assume the breath weapon hits two targets, and that they fail their saves, so the damage of the breath weapon is 35 acid damage multiplied by two targets for a total of 70 damage.
When we have a monster like this that has multiple attack forms, it's suggested to figure damage for three rounds of combat, and then average that damage. We should definitely do this with the neothelid.
This begs the question, how many times should we apply the breath weapon damage in the three rounds of combat? This isn't spelled out in the DMG, but if we look at the breath weapon recharge, we see it recharges on a 5 or 6 on a 1d6. This gives us odds of 1:3, which means we can assume the monster can use its breath weapon once per 3 rounds of combat.
The third way this monster can do damage is via its swallow ability. We can find this ability in the monster traits chart (which references a behir, btw). We're told to assume the monster swallows one creature and does two rounds of acid damage to it. The acid damage from the swallow is 35.
There are a few points to make here. The creature doesn't have multiattack, so it can only make one tentacle attack as an action. Using its breath weapon is an action, so on the turn it uses its breath weapon, it can't make a tentacle attack. Finally, a swallowed creature takes the acid damage at the start of the neothelid's turn, so the neothelid doesn't do this damage on the turn it swallows a creature, it does it on the neothelid's next turn.
So this is where some art comes into play, because there are basically two ways this could go.
The first, and most likely (and I think is the one the designers used) is this:
Round 1: Neothelid uses breath weapon. 70 damage (35 damage x 2 targets)
Round 2: Neothelid uses tentacle attack and swallows a creature. 33 damage
Round 3: Neothelid uses tentacle attack and deals damage to the creature swallowed in round 2. 68 damage (33 tentacle damage, 35 damage to swallowed creature)
Average damage per round = 70 + 33 + 68 = 171 / 3 rounds = 57 DPR
The second way this could go is this:
Round 1: Neothelid uses tentacle attack and swallows a creature. 33 damage
Round 2: Neothelid uses breath weapon and deals damage to creature swallowed in round 1. 105 damage (70 damage from breath weapon, 35 damage to swallowed creature)
Round 3: Neothelid uses tentacle attack and deals damage to creature swallowed in round 1. 68 damage
Average DPR = 33 + 105 + 68 = 206 / 3 rounds = 69 DPR
I think the first scenario is the most likely, and probably the way I would run this monster in combat. But we'll keep the second in mind to see how it impacts the CR.
So we have a damage output of 57 DPR, which gives us a beginning OCR of 9. I too looked at the attack bonus here. The expected attack bonus for OCR 9 is +7, however our neothelid has an attack bonus of +13. We increase the OCR by 1 for every 2 points the attack bonus is higher than expected. This would add 3 to our OCR, giving us an OCR of 12.
(Note: if we used the second damage output (69) calculation, we would've started with an OCR of 11 and would've ended up with an OCR of 14 due to the attack bonus.)
We then average the DCR of 17 and the OCR of 12, giving us a final CR of 15.
(Note: if we used the second damage output (69) we'd end up with a CR of 16 (average of DCR 17, OCR 14.)
So this is only two off of the CR 13 given in the book, but I take a look at what I did, and I see I made an error.
The attack bonus only applies to the tentacle attack. The tentacle attack is only providing 66 of our 171 total damage in those three rounds. The majority of the damage is actually coming from the breath weapon and the acid damage to the swallowed creature. Both of those rely on saving throws. The DMG tells us that if the monster relies more on effects with saving throws than on attacks to do its damage, we should use its save DC to figure the OCR instead of its attack bonus.
So let's return to our OCR calculation. We have a damage output (DPR) of 57, which gives us an OCR of 9. The save DC for OCR 9 is 16. The save DC for the neothelid's breath weapon and swallow damage is 18. So we add 1 to the OCR, because you add 1 for every 2 points the save DC is higher than what's listed for that OCR. That gives us an OCR of 10.
We now average the DCR of 17 and the OCR of 10, and we get 13.5. Now, I would think we'd round up to 14 here, but you can see that if we just round down instead, we get the CR of 13 that's in the book.
(Note: if we used the second damage output (69), we'd end up with an OCR of 11 and a final CR of 14.)
So did Wizards make a "mistake" in rounding the 13.5 down to a CR 13? I don't think so. I think this is another instance where the art of design (or perhaps a design guideline we're not privy to) came into play. I can only guess what that might be, however considering this creature doesn't have multiattack, doesn't have any resistances or immunities, and also considering that if the swallowed creature can do 30 damage to it, it's regurgitated out, I think a CR of 13 is more appropriate than 14 for this creature.