Level Up (A5E) Resistance/Immunity and CR

Reynard

Legend
I was reading through the MM monster design rules because I am planning on doing some design, and I discovered that no accounting was made for resistances and immunities in CR calculations. is this intentional? I feel like at some point resistance and/or immunity sort of counts as higher than suggested hit points and should require the same sort of balancing out. My gut says few, very specific resistances are probably not a concern, but something like "resistance to slashing, piercing and bludgeoning" is probably something like a 20% hit point boost. I'm not sure where "immunity to non magical weapons" would be (especially given the werewolf is CR 3 with 58hp which is right in the chart wheelhouse).

Thoughts?
 

log in or register to remove this ad


xiphumor

Explorer
I’ll also bump this.

While we’re here, if anyone know why encounter design doesn’t take the number of enemies into account, I’m also curious about that.
 

J.Quondam

CR 1/8
I'm not sure how directly applicable it is to A5e, but I'm guessing the CR logic behind it more or less still matches what's outlined here at Blog of Holding (scroll down to "monster traits")? There he specifically notes says that most resistances, etc, don't change CR much, but offers a rule of thumb to calculate it:
We can test common and seemingly powerful traits like legendary resistance and magic resistance and in almost all cases, the presence or absence of these traits has no correlation to higher or lower monster statistics. Therefore, they are not visibly affecting a monster’s CR. The only verifiable exceptions, as I mentioned here, are regeneration (which has a negligible but real effect, reducing some monster HP a by a few percent) and possession (which has a large effect, halving hit points) and possibly damage transfer. I think we can turn these three cases into a general rule: you may reduce damage-avoiding monsters’ hit points by the amount of damage you expect them to avoid over 3 rounds of combat.
(emphasis his.)
 


duneguy

Explorer
Hi! I do have some thoughts taht have been cooking in my head on this, which I'd love your thoughts on.

-Resistances and immunities - this is a tricky one: I do feel that they are relevant in CR at times, but most often their importance is much more negligible than you'd expect. I think I should have gone into this more and it might be something worth expanding further in further posts or publications.

Here's my thinking on why an immunity or resistance is fairly minor:

It's tough to design for all adventuring parties, since you don't know exactly how the party is composed, so I made some assumptions. One assumption is that half (or more) of the party will likely be making weapon attacks, while half (or less) will be dealing damage with energy attacks (fire, cold, and the other non-bludgeoning, piercing, and slashing damage types). There are 10 energy damage types, which means taht, on average, each energy type deals at most 5% of the expected damage in a generic encounter, probably less (1/10 of the 50% of damage not done by weapons). Let's say 4%, since I bet that b/p/s damage is actually higher than 50%, especially at low levels. So at first I'd expect a damage immunity to negate 4% of damage - that is, provide about the same benefit as a 4% increase in hit points.

However, it's probably less than this, since players are likely to avoid less-useful damage types, and most spellcasters have lots of damage type options to choose from. In order to avoid a damage type, they have to know about the immunity, which they probably already know or learn quickly. They may a) be using logic (I bet this frost giant won't take too much damage from my cone of cold) or previous experience (that poison spell didn't seem to do much to the last undead creature we met) or they may even learn during an encounter that a particular damage type is ineffective (hmm, my fireball didn't hurt the red dragon, I better not do that again). Therefore, an immune damage type is less likely to be used on a monster. Let's say that cuts down each damage immunity's benefit by half, down to 2%.

This above is all back-of-the-napkin math and based on assumptions about average party composition and so on, but that's the basis of my thinking. I think that an immunity is probably worth about the same as a 2% hit point boost, or 2 extra hit points for every 100 hit points a monster has. Resistance, of course, is worth half this: 1 extra hit point for every 100 hit points a monster has.

By this math, a single resistance or immunity to energy damage is just not enough to change a monster's CR.

Now if a monster has many immunities, that can build up. A ghost has 3 energy immunities and 4 resistances, which is worth taking into account in CR--especially since, while most spellcasters can easily avoid using a certain damage type, they may have difficulty avoiding all seven of a ghost's immunities and resistances. Using our 2%-1% rule of thumb above, we might say that all these energy protections together are worth about 10% extra hit points, or maybe a bit more since spellcasters might be forced to use an ineffective damage type even once they learn the ghost's resistances. Still, by CR math, a monster needs to gain 30 hit points in order to raise its CR by 1, and a 10% or 15% increase in hit points is not usually enough to do this until the monster gets to around CR 15 or higher. At this point making a rule like "7 or more energy immunities and resistances raise a monster's CR by 1 if it's CR 15 or higher" begins to seem like such a corner case it doesn't even seem worth noting.

Now it could be that my math/assumptions are wrong and the benefit of a single energy resistance/immunity is higher than I realize. But this is my thinking.

Immunity, or even resistance, to bludgeoning/slashing/piercing is different - that is quite valuable, since I think most parties do a ton of that type of damage. And I should have noted that such immunities/resistances are very valuable. But they are rare, and mostly for swarms. It's worth mentioning the much more common resistance: to nonmagic weapons. The value of this is entirely campaign- and narrator-dependent, so to figure out its worth we need to decide how common magic items (and the magic weapon spell, etc) are. I think that in a lot of campaigns, from character 1 to maybe 5 or 6, magic weapons are nonexistent or rare, and resistance to nonmagic weapon damage is very valuable - worth, perhaps, as much as 50% more hit points at CR 1, and decreasing rapidly. By the time you get to, say CR 7, I'd think that it's of almost trivial benefit in many campaigns. (But then, what do I know about most campaigns? I don't have as much data as I'd like.) If I were to revise the monster creation guidelines, I think I would note the value of resistance to nonmagic b/p/s damage at low CR.

-Number of monsters: why doesnt encoutner building take number of monsters into account?

In the 5e DMG, a number-of-monsters qualification was added to their formula, adding a multiplier to the monster's total XP if there are a lot of them - which was necessary in that system, since their method of encounter planning - totaling XP - is exponential and undervalues low-CR monsters. Since we use a totally different way to plan encounters - adding CR, which is more linear - we don't need to use this multiplier as a fix. We already believe large numbers of low-CR monsters are much more dangerous than the DMG does! (Also, WOTC seem to have dropped this multiplier in Xanathar's encounter building as far as I can tell.)
 

dave2008

Legend
Hi! I do have some thoughts taht have been cooking in my head on this, which I'd love your thoughts on.

-Resistances and immunities - this is a tricky one: I do feel that they are relevant in CR at times, but most often their importance is much more negligible than you'd expect. I think I should have gone into this more and it might be something worth expanding further in further posts or publications.

Here's my thinking on why an immunity or resistance is fairly minor:

It's tough to design for all adventuring parties, since you don't know exactly how the party is composed, so I made some assumptions. One assumption is that half (or more) of the party will likely be making weapon attacks, while half (or less) will be dealing damage with energy attacks (fire, cold, and the other non-bludgeoning, piercing, and slashing damage types). There are 10 energy damage types, which means taht, on average, each energy type deals at most 5% of the expected damage in a generic encounter, probably less (1/10 of the 50% of damage not done by weapons). Let's say 4%, since I bet that b/p/s damage is actually higher than 50%, especially at low levels. So at first I'd expect a damage immunity to negate 4% of damage - that is, provide about the same benefit as a 4% increase in hit points.

However, it's probably less than this, since players are likely to avoid less-useful damage types, and most spellcasters have lots of damage type options to choose from. In order to avoid a damage type, they have to know about the immunity, which they probably already know or learn quickly. They may a) be using logic (I bet this frost giant won't take too much damage from my cone of cold) or previous experience (that poison spell didn't seem to do much to the last undead creature we met) or they may even learn during an encounter that a particular damage type is ineffective (hmm, my fireball didn't hurt the red dragon, I better not do that again). Therefore, an immune damage type is less likely to be used on a monster. Let's say that cuts down each damage immunity's benefit by half, down to 2%.

This above is all back-of-the-napkin math and based on assumptions about average party composition and so on, but that's the basis of my thinking. I think that an immunity is probably worth about the same as a 2% hit point boost, or 2 extra hit points for every 100 hit points a monster has. Resistance, of course, is worth half this: 1 extra hit point for every 100 hit points a monster has.

By this math, a single resistance or immunity to energy damage is just not enough to change a monster's CR.

Now if a monster has many immunities, that can build up. A ghost has 3 energy immunities and 4 resistances, which is worth taking into account in CR--especially since, while most spellcasters can easily avoid using a certain damage type, they may have difficulty avoiding all seven of a ghost's immunities and resistances. Using our 2%-1% rule of thumb above, we might say that all these energy protections together are worth about 10% extra hit points, or maybe a bit more since spellcasters might be forced to use an ineffective damage type even once they learn the ghost's resistances. Still, by CR math, a monster needs to gain 30 hit points in order to raise its CR by 1, and a 10% or 15% increase in hit points is not usually enough to do this until the monster gets to around CR 15 or higher. At this point making a rule like "7 or more energy immunities and resistances raise a monster's CR by 1 if it's CR 15 or higher" begins to seem like such a corner case it doesn't even seem worth noting.

Now it could be that my math/assumptions are wrong and the benefit of a single energy resistance/immunity is higher than I realize. But this is my thinking.

Immunity, or even resistance, to bludgeoning/slashing/piercing is different - that is quite valuable, since I think most parties do a ton of that type of damage. And I should have noted that such immunities/resistances are very valuable. But they are rare, and mostly for swarms. It's worth mentioning the much more common resistance: to nonmagic weapons. The value of this is entirely campaign- and narrator-dependent, so to figure out its worth we need to decide how common magic items (and the magic weapon spell, etc) are. I think that in a lot of campaigns, from character 1 to maybe 5 or 6, magic weapons are nonexistent or rare, and resistance to nonmagic weapon damage is very valuable - worth, perhaps, as much as 50% more hit points at CR 1, and decreasing rapidly. By the time you get to, say CR 7, I'd think that it's of almost trivial benefit in many campaigns. (But then, what do I know about most campaigns? I don't have as much data as I'd like.) If I were to revise the monster creation guidelines, I think I would note the value of resistance to nonmagic b/p/s damage at low CR.

-Number of monsters: why doesnt encoutner building take number of monsters into account?

In the 5e DMG, a number-of-monsters qualification was added to their formula, adding a multiplier to the monster's total XP if there are a lot of them - which was necessary in that system, since their method of encounter planning - totaling XP - is exponential and undervalues low-CR monsters. Since we use a totally different way to plan encounters - adding CR, which is more linear - we don't need to use this multiplier as a fix. We already believe large numbers of low-CR monsters are much more dangerous than the DMG does! (Also, WOTC seem to have dropped this multiplier in Xanathar's encounter building as far as I can tell.)
Paul,
I notice in the new monster building tool that: "...Most languages, senses, skills, saving throw proficiencies, movement forms, and traits don’t require a change to game statistics."

Emphasis mine. That is not true in the DMG guidelines and not true from an actually in play perspective. Just curious why/how you came to your conclusion. Thank you!
 

Stalker0

Legend
Paul,
I notice in the new monster building tool that: "...Most languages, senses, skills, saving throw proficiencies, movement forms, and traits don’t require a change to game statistics."

Emphasis mine. That is not true in the DMG guidelines and not true from an actually in play perspective. Just curious why/how you came to your conclusion. Thank you!
yeah I generally feel this is one of the few things that matters more at high levels. At low levels, a saving throw proficiency is +2....meh, big whoop, not going to change the math enough to really matter on the CR.

At 17th level, that's a +6....that's a big deal, that's a stronger bonus than magic resistance for that save.
 

lichmaster

Adventurer
@duneguy Another consideration about spells/innates that may affect CR: creatures that can cast Polymorph can turn into a beast with CR up to their original CR. Assuming both the original creature and the beast have the standard amount of hp given the CR, this could potentially double its effective hp. In a5e polymorph also changes the mental scores of the original creature, thus it may not be a good deal, but in a potentially deadly situation this kind of power can make a huge difference.
How would you assess this?
 

Morrus

Well, that was fun
Staff member
Paul,
I notice in the new monster building tool that: "...Most languages, senses, skills, saving throw proficiencies, movement forms, and traits don’t require a change to game statistics."

Emphasis mine. That is not true in the DMG guidelines and not true from an actually in play perspective. Just curious why/how you came to your conclusion. Thank you!
I don't want to talk for him, but I assume it's because the revised stats in Paul's chart in the MoMe assume that the creature has the appropriate proficiency (and LU monsters tend to have more saving throw proficiencies).
 

duneguy

Explorer
@lichmaster absolutely, polymorph should have a big impact on CR just like damage transference does! I probably didn't think of it because I don't give it out much - though I give out shapeshifting a lot.

@dave2008 you and Morrus are both right on the question of saving throw proficiencies. And saving throw proficiencies really merit a more complete treatment than I gave them. On average and without checking, I bet I give out at least a saving throw proficiency at tier 2, two at tier 3, and three at tier 4 to most monsters. Rather than changing CR for each saving throw proficiency, I might want to set some expectation like that for the number of saving throw proficiencies a monster should have by CR - with maybe a -1 CR adjustment if you have far too few.
 

lichmaster

Adventurer
@lichmaster absolutely, polymorph should have a big impact on CR just like damage transference does! I probably didn't think of it because I don't give it out much - though I give out shapeshifting a lot.
@duneguy I gave another thought to this aspect, which never occourred me before.
Given that in 5e polymorph effectively gives extra hp, a creature that can cast the spell multiple times can have many times over its normal hp.
So, as a follow up to the CR adjustment, I think one should also consider how many times the creature can polymorph.

Even if the damage over the normal amount of hp of the polymorphed creature goes to its normal shape, the spell is still a huge buff, up to the point I'm considering this kind of nerf:
- instead of simply assuming the hp of the new form (i.e at full health), the polymorphing character will assume the new form's hp minus the damage already taken (i.e. the damage carries over both ways).
I'll create a separate post with an analysis of this modification.
 

Dungeon Delver's Guide

An Advertisement

Advertisement4

Top