D&D 5E Spells and Evaluating Monster CR

[Quickleaf]

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.

I appreciate you sharing your approach. I'm going to give it a try, and see what shakes out...

Defensive CR 9
eff HP = 117 * 1.5 (for target CR 5-10) = 175
eff AC = 16 + 2 saves + 2 magic resist = 20

or

Defensive CR 8
eff HP = 117 * 1.25 (for target CR 11-16) = 146
eff AC = 16 + 2 saves + 2 magic resist = 20

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.

Ok, hypothetical 3 round combat means prioritizing 8th, 7th, and 6th level spell slots. Her spells are from bard & cleric lists, so going to stick to those. Harm (6th) (14d6 or 49), Firestorm (7th) (7d10 or 38 vs. two targets = 76), and since there's no damaging bard or cleric spell at 8th, I'll just go with another Firestorm I guess (76).

(49 + 76 + 76) / 3 = 67 DPR
Spell save DC is ~18
That gives me a data point Offensive CR = 11

And (9 defensive CR + 11 offensive CR) / 2 = data point CR 10.

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.

I'm going to assume average HP for a 10th level party is something like a d8, maximum at first, high average each level thereafter, with CON 14. So it would = 8 + (9 * 5) + (10 * 2) = 73 hit points.

She can cast two plane shifts with her 7th level and 8th level spell slots. Maybe I can equate an upcast 5th level hold person or dominate person as being roughly equivalent to a third plane shift? Maybe? I don't know. I'm going to do that just for the sake of completing the exercise.

That dives her a DPR of 73.

Which the magic on the DMG (page 274) tables gives us Offensive CR = 11.

Same as above, (9 + 11) / 2 = Total CR 10.

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)!

I'm going to assume average HP for a 9th level party is calculated same as I did above. So it would = 8 + (8 * 5) + (9 * 2) = 66 hit points.

DPR 66. Which gives us Offensive CR = 11

And again, Total CR 10.

However... These "offensive" tactics are also about control, so they're reducing the overall damage/offense output the PCs can bring to bear against the monster. And somehow I'd need to account for that in her Defensive CR calculations, right?

 

[NotAYakk]

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?

Honestly, it is just "fireball is a nasty spell. So is hold person."

Both are strong top-tier spells. Hold person is an upcast lower level spell, but it is overpowered in the hands of monsters, because all PCs are persons.

Planeshift and Disintegrate produce similar levels of dread. They are both save or be gone for good at around this level; disintegrate requires prior damage on some targets, planeshift an attack and a save.

...

So the thing is that offence and defence are fungible under CR calculations. Preventing X damage and dealing X damage are very similar in value. I mean, not identical, but similar.

As 5e monsters are pretry linear (well affine) in danger with CR, getting a 12 or a 13 or a 11 CR calculation really doesn't matter. That is a difference of under 10% threat. (CR 1/2 to 1 to 2 is larger percentage wise by far, which is why mobs of monsters tend to be a delecate problem; rounding/errors in them in theory matter more)

The practical goal is to avoid a CR 25 monster or CR 1 glass cannon when you want a CR 12 challenge, not avoid an accidental CR 14.

 

[aco175]

I also go with DM cheats when using homemade monsters. We had this discussion a couple months ago, but I will add or subtract powers and HP is needed to make the fight better for the players. If I think that the monster needs multiattack- I give it in the middle of the fight, same with things like clerics or shamans being able to cast lightning bolt.

An example of this is an epic fight with the elemental lord Olhydra a few weeks ago. They players encountered her in the water lake in the elemental evil storyline and were water walking at the time. There was a lair action to create a freezing cloud that I changed to make ice in a 30x30ft area on the lake. This effectively made the PCs frozen on the ice using an action to free themselves instead of the obscuring cloud. I made it up, but think that the combat was better for the players at the time.

Your monster here can be great on paper and you do not know how it actually is until it runs into the players. If you prepare lair actions and such, you can always take them away. I do not have the time to stage mock combats and play with charts, so I mostly wing it. I always thought that @Quickleaf was one of the better designers on the boards and doesn't need too much help. That sounds cheesy, crap.

 

[NotAYakk]

Also, I'll plug this:

D&D 5E - Ridiculously Simple Monster Building

People complain that the DMG CR stuff doesn't work. And it really doesn't work for the use most people put it to. So here is a system based off the DMG CR stuff, but upside down. Instead of starting with stats and generating CR, this starts with CR and gives you a budget to buy stats. The...

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using that I get:
HP 117 (Resists: worth about +25% say) 18.3 MBP
AC 16 (+2 for magic resistance) 8 MBP
Attack Stat: +6 (weapon attacks are fluff-level honestly; she should only use them on a held solo foe) 6 MBP
Attack Damage: 14 4.7 MBP

Base: 37 MBP

Hold person, Plane Shift and Hold Person are likely spells to use (assuming she has minions). I value those at 105 105 and 70 HP each (upcast fireballs and a disintegrate roughly); she has the slots to attempt a hold on most of the party. .

(If fought without minions, this monster lacks the ability to actually kill PCs.)

238 "single use" damage after subtracting the 14 "at-will" she is forgoing per attack), divided by 9 is 26.4 MBP

Total 63.4 MBP.

We could toss it a few more points for stuff like death ward, but that would be rounding error really.


A CR 11 monster has 44+2+14=60 MBP
A CR 12 monster has 48+2+14=64 MBP

So CR 11 or 12.

 

[Sword of Spirit]

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

Just a quick note. Planeshifting back won’t get you back in the fight unless you are fighting at a teleportation circle, because it has imprecise targeting. The DMG explains it a bit better than the spell description, but basically you might be miles away, and it would pretty rare (IMO) for it to place you within standard combat range of your destination.