D&D 5E Spells and Evaluating Monster CR

[Quickleaf]

When homebrewing a spellcasting monster, how can I account for spells that don't have an obvious and/or easily calculated contribution to defensive/offensive challenge in determining the monster's CR?

As a quick (hopefully unnecessary) disclaimer: I'm totally aware of the shortcomings of CR. Treat this as a question concerning monsters for publication, not monsters I'm slap-dashing together for a home game. In other words, work from the assumption that some sort of evaluation of the monster's relative threat level needs to be provided in its stat block, and that CR – while flawed – is the lingua franca to communicate that. Hopefully, even if you feel compelled to challenge the premise or point out the inherent flaws in the system, you can also contribute something towards the question I'm eager to know about. Thanks.

While this question has come up for me before, I've usually been able to work through some maths (e.g. mirror image) or use a "good enough" rule-of-thumb / DM feel, but the current monster I've been working on has me scratching my head. None of her spells deal obvious damage and only one of her spells (shield of faith) contributes to obvious defense, but that's at the bottom of her spellcasting priorities considering she has much higher level spells. Yet some of these spells have the potential to be game changers if cast at the right moment, and clearly a monster with these spells is more challenging than a monster without them.

I've included the monster in spoilers (with DNDBeyond links to spells), if you'd like to engage with the example I'm working on -- a transcended succubus aka "lilitu" turned "high priestess" of Graz'zt. Her defensive CR is roughly 9, but I'm flummoxed on how to estimate an offensive CR. To make it easier, I've striked-through spells that have long casting times or which are clearly intended for social/exploration scenes (e.g. detect thoughts & sending).

But feel free to use your own examples! Principles & troubleshooting specific spells are what I'm most interested in, and are what I imagine would be most helpful to other board members.

Spoiler: Lavendeth, the spellcasting monster with a perplexing CR

LAVENDETH
Medium fiend (demon), chaotic evil
Armor Class 16 (natural armor)
Hit Points 117 (18d8+36)
Speed 40 ft
STR 10 (+0) DEX 19 (+4) CON 15 (+2) INT 16 (+3) WIS 16 (+3) CHA 22 (+6)
Saving Throws Con +?, Int +?, Wis +?, Cha +?
Skills Deception +?, Insight +?, Intimidation +?, Perception +?, Persuasion +?, Religion +?, Stealth +?
Damage Resistances cold, fire, lightning; bludgeoning, piercing, and slashing from nonmagical attacks
Damage Immunities poison
Condition Immunities poisoned
Senses darkvision 60 ft., passive Perception 11
Languages All, telepathy 60 ft.
Challenge ? (? XP) Proficiency Bonus +?

Innate Spellcasting. Lavendeth’s innate spellcasting ability is Charisma (spell save DC ?). She can innately cast the following spells, requiring no material components:
At will: charm monster, detect thoughts, disguise self, fly, suggestion
1/day each: dominate person, mass suggestion

Magic Resistance. Lavendeth has advantage on saving throws against spells and other magical effects. However, a cleric or paladin who recognizes her true nature bypasses her Magic Resistance.

Shroud Presence. Lavendeth cannot be detected by abilities or spells that sense demons or fiends. Additionally, detect magic never reveals a magical aura from an illusion spell around her, even when she has cast disguise self.

Spellcasting. Lavendeth is a 16th level spellcaster. Her spellcasting ability is Charisma (spell save DC ?). She has the following spells prepared:
Cantrips (at will): guidance, mending, resistance, spare the dying, thaumaturgy
1st level (4 slots): command, cure wounds, detect magic, shield of faith
2nd level (3 slots): enthrall, hold person, silence
3rd level (3 slots): bestow curse, dispel magic, nondetection, sending
4th level (3 slots): confusion, death ward, freedom of movement
5th level (2 slots): mass cure wounds, planar binding, raise dead
6th level (1 slot): Otto's irresistible dance, planar ally
7th level (1 slot): divine word, plane shift
8th level (1 slot): glibness

Telepathic Bond. Lavendeth ignores the range restriction on her telepathy when communicating with a creature she has charmed. The two don't even need to be on the same plane of existence.

ACTIONS
Multiattack. Lavendeth makes a claw attack and stinger attack.
Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 7 (1d6+4) slashing damage.
Stinger. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 7 (1d6+4) piercing damage and 7 (2d6) poison damage, and the target’s Wisdom score is reduced by 1d4. The target becomes a mindless slave to Lavendeth if this reduces its Wisdom to 0, obeying her every whim. Only a wish spell can restore a creature made into her mindless slave. Otherwise, the reduction lasts until the target finishes a short or long rest.
Lavendeth’s Gift (1/day). Lavendeth touches a willing or incapacitated creature, bestowing it with a magical charm for 24 hours or until dispel magic is cast on the creature. For the duration, Lavendeth’s name (written in Abyssal) appears as a tattoo somewhere on the creature’s body and the creature detects as a fiend to all abilities or spells that sense fiends. Additionally, while the charm is active, the creature gains advantage on its Charisma checks and all its saving throws. While the charm remains, Lavendeth can communicate telepathically with the creature even across planes, she has a sense of the creature’s general well-being, and as an action she can see and hear through the creature (becoming blinded and deafened to her own surroundings). Resuming her own senses requires a bonus action.

 

[aco175]

I tend to find that PCs can tackle more than what's presented and go with my gut on monster CR when making them. I was thinking 9-10 looking at her abilities. I was actually thinking of increasing some of her attacking power.

 

[NotAYakk]

Simulate a 3 round combat. Have her go all out.

In those 3 rounds, how much damage did she do? If she did a control effect "as good as" damage (say, a 5 person hold person), decide what damage it is as good as and use that.

Then divide by 3.

That is it. Dispell magic or the like is assumed to fall out in the rounding.

 

[jgsugden]

Where would I put the CR for this creature? 12.

How do I reach that decision? I started by taking a blind guess at the right power level and comparing it to the monsters I find there. So, it is about 20% comparing it to other CR 12 monsters like the Archdruid, and Syndra Silvane and 80% not worrying if it would be better off at 10, 11, 12 or 13 ... because I'd use any of them the same way in a game / encounter building... and I use milestone advancement, so I don't care how many XP the party gets from fighting it. To that extent, it does not matter.

Most of the monsters I create do not get an accurate CR - they get built and used. If built on pen and paper, they get no CR. If built in a tool that assigns a CR, I often just don't bother setting it.

 

[Quickleaf]

I tend to find that PCs can tackle more than what's presented and go with my gut on monster CR when making them. I was thinking 9-10 looking at her abilities. I was actually thinking of increasing some of her attacking power.

I don't meant to press if you're gauging it intuitively, but could I ask how did you reach that CR 9 to 10 estimation?

You don't come out and say this, but I'm going to ask: Are you implying that if it's hard to calculate a monster's offensive challenge – or if calculating its offensive challenge seems unclear or muddied – that is an indication of a design shortcoming that should be addressed?

Simulate a 3 round combat. Have her go all out.

In those 3 rounds, how much damage did she do? If she did a control effect "as good as" damage (say, a 5 person hold person), decide what damage it is as good as and use that.

Then divide by 3.

That is it. Dispell magic or the like is assumed to fall out in the rounding.

Going all out for Lavendeth would probably include fighting with lots of minions, upcasting hold person, forcible plane shifting, and potentially confusion.

Taking your example of an upcast hold person, how would you decide what damage that is "as good as"?

Correct me if I'm making a false assumption about what you mean, but wouldn't this mean dividing average Hit Points of a PC by 3, then multiplying by number of PCs (of which D&D assumes 4)? Since we assume 3 rounds of combat, and putting a PC out of commission is 1/3rd of that combat. But doesn't that require an assumption about what level PCs are facing the monster... which makes its offensive CR a moving target that scales according to the party?

Where would I put the CR for this creature? 12.

How do I reach that decision? I started by taking a blind guess at the right power level and comparing it to the monsters I find there. So, it is about 20% comparing it to other CR 12 monsters like the Archdruid, and Syndra Silvane and 80% not worrying if it would be better off at 10, 11, 12 or 13 ... because I'd use any of them the same way in a game / encounter building... and I use milestone advancement, so I don't care how many XP the party gets from fighting it. To that extent, it does not matter.

Most of the monsters I create do not get an accurate CR - they get built and used. If built on pen and paper, they get no CR. If built in a tool that assigns a CR, I often just don't bother setting it.

Funny, when I'd originally written the monster, I put down CR 12 without having done any of the maths.

I understand the "it does not matter / no worries" perspective. Given that is the majority of how you approach monsters, you may not wish to answer further – and I totally understand that – but would you be willing to elaborate on how your comparison process looked for my example monster Lavendeth? You said you compared her to the Archdruid with spells like firestorm and sunbeam, and Archmage with spells like cone of cold and globe of invulnerability. How do you compare Lavendeth to those?

 

[aco175]

I don't meant to press if you're gauging it intuitively, but could I ask how did you reach that CR 9 to 10 estimation?

You don't come out and say this, but I'm going to ask: Are you implying that if it's hard to calculate a monster's offensive challenge – or if calculating its offensive challenge seems unclear or muddied – that is an indication of a design shortcoming that should be addressed?

I tend to think the level of the PCs when they fight a monster. Would my party of 10th level PCs be challenged by this? My players are not making PCs that optimized, but would likely kill this monster in 3-4 rounds with her having a few minions with her. Give her some lair powers and such, maybe add some to multiattack and I can see CR12.

Also, looking at the CHA22 for +6. This makes her save DC around 18/19, making it hard for fighters and such to save from charms. Spells are swingy in my opinion and make if feels like a higher CR is warranted and it could be depending on how the saves go. One or two of the PCs could be taken out fast and now the challenge is a lot more for a group of 10th level PCs, but could that be the same for 14th level PCs with only another +1 to the saves?

Overall I would boost a few combat abilities to take a hit or deal more area damage, add some lair actions and call her CR12.

 

[NotAYakk]

Hold person? Easy way is to say it is a strong offensive spell on par with a fireball. So like 105 damage when upcast.

(Hopefully a PC saves and breaks her comcetration or dispells it).

Planeshift-touch is on par with disintegrate. Call it 75 damage.

Heals are stupid to do in combat usually, especially when you should be killing stuff. So ignore that.

 

[Sword of Spirit]

Basically, “How do save or suck/die spells affect CR?”

It does look pretty darn hard. There isn’t an obvious straightforward method of figuring it out, though there probably should be.

My next step would be to swap out her prepared spells with blasting spells and see what that does to her CR just to get a data point. Then, pretend she’s fighting a party of 4 with average level equal to that CR and see how many effective hit points she negates by taking out characters with removal spells like planeshift. I would say something like dominate would only last 2 rounds (but it might also contribute to her damage output during that time). Figure out an offensive CR from there. Then, I’d run the same numbers against a level 9 party (because that’s her defensive CR so it’s another potential starting point).

Then I‘d take each offensive CR and find a final CR. If the results with both data points are really close, then I would look at it based on feel. If they were only one apart, well, it’s one or the other. If they were two apart, then go for the middle (unless my gut says to round high or low). And it’s possible they could end up being the same number (in which case I have my answer)!

 

[Quickleaf]

Hold person? Easy way is to say it is a strong offensive spell on par with a fireball. So like 105 damage when upcast.

(Hopefully a PC saves and breaks her comcetration or dispells it).

Planeshift-touch is on par with disintegrate. Call it 75 damage.

Heals are stupid to do in combat usually, especially when you should be killing stuff. So ignore that.

What I like about your approach is that you're creating benchmarks for spells – which is something replicable by other DMs working on their own monsters.

Would you be willing to walk me through how you're reaching those equivalencies?

How is upcast hold person on par with fireball?

Hold person needs to be cast with a 5th level spell slot to get the hypothetical 4-person party. That means a 9th level spellcaster like a CR 6 mage. It's denying actions, so it's actually reducing the party's capability to dish out damage/effects, which means it's effectively increasing the monster's defenses, right?

How is planeshift on par with disintegrate?

I can planeshift a 9th level rogue with 57 hit points or a 16th level fighter with 148 hit points, and if that PC has no means of easy planar travel they're out of the fight. Similarly, I can planeshift a 13th level wizard with 67 hit points, but if that PC can cast planeshift, they can be back in the fight in a round (so effectively – in the hypothetical 3-round combat – I did 67 / 3 = 23 damage). But those HP values and hence damage estimates are all over the place, right?
EDIT: Strangely, if I take the average of those three damage values I pulled out of the air (57 + 148 + 23), I get 76, which is pretty much the average damage value for disintegrate.

 

[Quickleaf]

I tend to think the level of the PCs when they fight a monster. Would my party of 10th level PCs be challenged by this? My players are not making PCs that optimized, but would likely kill this monster in 3-4 rounds with her having a few minions with her. Give her some lair powers and such, maybe add some to multiattack and I can see CR12.

Also, looking at the CHA22 for +6. This makes her save DC around 18/19, making it hard for fighters and such to save from charms. Spells are swingy in my opinion and make if feels like a higher CR is warranted and it could be depending on how the saves go. One or two of the PCs could be taken out fast and now the challenge is a lot more for a group of 10th level PCs, but could that be the same for 14th level PCs with only another +1 to the saves?

Overall I would boost a few combat abilities to take a hit or deal more area damage, add some lair actions and call her CR12.

Thank you for your feedback!

Seems like there's a bit of "chicken or the egg" question when it comes to level of PCs vs. offensive CR of unusual monsters like this. In order to evaluate the offensive CR you need to have some assumptions in mind about the level you anticipate the PCs being when they tangle with the monster.

Yeah, you honed in on exactly the "moving target" issue I was wondering about. Control spells like plane shift, dominate person, or hold person targeting saves seem like they can be almost equally effective against lower or higher level PCs.