In the Dungeon Master's Guide, there's a table I make a lot of use of - on page 274, it's called "Creating Quick Monster Stats." While the table's purpose is meant to give a quick way to conjure up a monster by just going to the appropriate CR row and using the stats found there, it's also used as a tool in tuning a monster, but through a more complicated process that continues on later pages.
The table is useful, though it has some oddities. For example, the AC column only goes up to 19 - even as the CR rises to 30. I cannot imagine any monster with a CR of, like, 25 or higher having less than 20 AC, so this is, of course, an imperfect tool. Surely just because something is wearing plate armor and a shield doesn't put it outside of the computations to create a CR. But in such cases, you might just have to wing it.
The thing is, I've actually found this table and the process that the DMG describes to be pretty useful in creating fairly balanced monsters that are good for the CR they earn.
However, in interviews, the folks at WotC said that in their earlier books, their monsters didn't need to meet all those requirements, which might account for why sometimes, a monster's CR doesn't always really match up with expectations.
I wanted to see if some of the new monsters did wind up meeting these standards. I'm going to try to go somewhat middle-of-the-road here (I'd initially thought I'd go for Baphomet, the demon lord, but let's build to that). One of the big goals of MotM was to make spellcasting characters have more straightforward options for damage-dealing.
The Alhoon is an undead Mind Flayer who has become an outcast for practicing arcane magic, rather than relying on their natural psionics. The original Alhoon from Volo's Guide to Monsters had a big mess of Wizard spells and spell slots. In the revision, their "spellcasting" feature has been simplified to a number of at-will spells and a few spells that can be cast once per day - none of which do damage.
For damage output, they have a classic Mind Flayer's Mind Blast, as well as two types of attacks - the melee Chilling Grasp attack or the ranged Arcane Bolt, each of which they can do twice per round.
The DMG rules have us calculate two values: Offensive CR and Defensive CR (what I abbreviate to OCR and DCR). Each of these is based on averages of where various values they have land on the aforementioned monster stat table - for example, if their effective HP (we'll get into that shortly) lands at CR 7 and their AC lands at CR 5, we consider their DCR to be 6.
The Alhoon is listed as CR 10. So we'll ultimately want the OCR and DCR to average out to that.
Let's start with their DCR. To do this, we need to look at their AC and their effective HP.
Effective AC is a little complex - we need another table, found on page 277. Based on the expected CR of the monsters, we see how resistances and immunities to certain damage types will affect the "effective" health of the monster - after all, if it's only taking half the damage it normally would, it's like it has twice the health. In the Alhoon's case, we have resistance to cold, lightning, and necrotic damage, and immunity to poison.
There is definitely some flexibility here, of course. If the monster is resistant to acid damage, that might not even come up in combat. However, if it's resistant to bludgeoning, piercing, and slashing damage from nonmagical attacks, that's more likely to be a thing.
In this case, I think that with cold and lightning being somewhat common damage types, and necrotic particularly common for folks like Warlocks and Clerics, I'd say this counts. However, while monsters do a ton of poison damage, I think players do so less often, so I'm not really going to account for the poison immunity. Thus, in this case, we'll say it has the resistance multiplier, which increases its effective HP by 50%.
The Alhoon's average hit points are 150, so we'll treat its effective HP (or EHP) as 225. That lands us in the range for CR 11. Now, it's AC is 15. That could fit in CR 5, 6, or 7. Given that the 225 is toward the lower end of its range for CR 11 (221-235) I'm going to skew this downward, saying the AC is earning the CR 5 rank. We the CR 11 we get for EHP with the CR 5 we get for AC and land at a total DCR of 8.
Ok, so damage needs to step up to justify a CR 10 - we're hoping it'll be an OCR of 12 to average out to that.
The Alhoon has three damaging options that are going to be relevant in different situations. Its big hit is Mind Blast, but this is a recharge ability. It recharges on a 5 or 6, making it usable in about one out of every three turns. This complicates things.
To calculate OCR, we need to use the monster's average damage output per round (if it hits with its abilities,) finding the CR earned by that value, and the averaging it with the value earned by its chance to hit. Again, the Alhoon complicates things for us, because its "chance to hit" is a saving throw DC in one case and an attack bonus in the others - and even though both are based on the Alhoon's Intelligence, the DC of 16 is considered a lower CR value than the hit bonus of +8.
Let's save that for later. Now, let's do damage. We want to calculate how much damage the thing will do on an average turn. To do so, we're going to average it out over three rounds of combat - one with the Mind Blast, and one with the attacks.
The Mind Blast is an area-effect ability, so we're going to treat it as if it's hitting two targets. It does an average of 22 damage (and stuns the target, which might skew it upward if we need a tiebreaker) so we're going to count this as 44 damage for that first round.
The attacks, which it does twice per turn, actually do different amounts of damage (sigh, this wasn't the easiest monster to choose for this). I'm going to say we'll act as if it's not in melee on turn 2 and then it is on turn 3, so we'll just average them.
Its Arcane Bolt does an average of 28 damage on a hit, so two means 56 damage.
Chilling Grasp does 14 (and heals it for 14, which, again, complicates this) for a total of 28 damage per round.
So, we'll average the damage here, and find that on a typical turn, the Alhoon does about 43 damage per turn. That's a CR 6 damage output, but we might skew it a bit higher given the extra effects.
Now, its attack bonus is +8, which can fit in the 11, 12, and 13 CR rows, while its save DC is 16, which can be in 8, 9, and 10. Given the potential to stun, and thus get advantage on rolls, I'm going to be generous and put that at CR 12 levels.
Thus, the OCR looks to be about 9. With a DCR of 8, that winds up being a bit lower than what we were hoping for. I'll grant that the healing from Chilling Grasp and the stun from Mind Blast (which could, of course, hit more than two targets) might push it up farther.
Perhaps I chose the wrong monster to test this on. Let me try to go for something a lot simpler.
Enchanter Wizard has no resistances or immunities, and while its Instinctive Charm bonus action could be considered an inflation of its DCR, I think we'll have an easier time here. We're just going to use its Arcane Bolt attacks to calculate OCR. So:
DCR:
We're going to assume they've got Mage Armor up at all times, giving them an AC of 15, and their average HP is 49. While 49 HP is only enough to get them a CR of 1/4, 15 AC puts them in the 5,6, or 7 AC range. When dealing with the fractional CRs, rather than truly averaging them, I just count the row that is directly between them. 49 is the very top of the CR 1/4 range, so we're going to count from the CR 7 row regarding AC, giving us a DCR of 3.
OCR:
The Enchanter's only offensive abilities are Arcane Bolt, which deals 19 (average) psychic damage and has a +6 to hit. It does three of these, so we multiply the 19 by that - giving us 57 damage per round. 57 puts us in the bottom of the CR 9 range. Then, +6 can be in the CR 5, 6, or 7 range. Given that 57 is the very bottom, we'll go with 5. Thus, between the damage and the bonus to hit, we're looking at a DCR of 7.
And wouldn't you know it? With a DCR of 3 and an OCR of 7, that averages out to a CR of 5 - which is what the Enchanter Wizard is listed as. Clearly a bit of a glass cannon, but it has the cannon to back it up.
Obviously, the more complex a stat block, the trickier, more subjective it is to use the table. But with a little finesse, it's a pretty useful tool
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