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Suppose we took a normal monster - say, perhaps, a lion (CR 1) - and gave it resistance to non-magical slashing, piercing, and blunt damage. How much would this affect the threat to a party?

I'm hoping for a relatively quantitative answer. Obviously, it will become more dangerous, but that vague notion doesn't really help my encounter design. Nominally, a pair of lions is a moderate threat to a 4 player party of 3rd level characters. With this buff, would they be deadly? Would they be dangerous to a 4th level party?

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It generally increases defensive challenge rating by 2 to 6, so final challenge rating goes up 1, 2, or 3...

The DMG guidance for creating a custom monster with resistances states:

For example, a monster with an expected challenge rating of 6, 150 hit points, and resistance to bludgeoning, piercing, and slashing damage from nonmagical weapons effectively has 225 hit points (using the 1.5 multiplier for resistances) for the purpose of gauging its final challenge rating.

So to determine defensive challenge rating, we see:

Read down the Hit Points column of the Monster Statistics by Challenge Rating table until you find your monster’s hit points. Then look across and note the challenge rating suggested for a monster with those hit points.

Since the proposed resistances means we multiply effective hit points by 1.5, just eyeballing the defensive challenge rating table gives us a DCR increase of +2 on the lower end, up to +6 on the higher end. Since resistances don’t affect Offensive CR at all, and final CR is the average of DCR and OCR, +2 to +6 DCR yields +1 to +3 final CR. The more hit points the monster has, the more significant of an effect adding resistance will have.

Finally, it must be observed that if the whole party can ignore the particular resistance, CR doesn't change at all. Phillipp's answer gives a robust discussion of this.

...except when defensive challenge rating is really low.

Now, when the CR is really low, like less than 1, it does get a little more difficult, as is the case with the lion mentioned in the question. Going off of hit points, the Lion's defensive CR comes out to 1/8, but because it has pack tactics and can possibly make two attacks per turn, its offensive CR comes out to 2, which is where the CR 1 final calculation comes from. Adding 50% on to the lion's effective HP only brings its DCR up to 1/4, which would still average out to CR 1 in the end. The method outlined in the first section seems to work just fine when working with DCRs of one or more, but the fractional challenge ratings complicate the averages and can lead to "no change" when the resistances are applied to monsters that already had really low DCRs.

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  • \$\begingroup\$ Hi Thomas, I did not look at the tables yet, but the +1/+2 to the final CR is for the example in the question (the lion) or is more general and applies to each monster? \$\endgroup\$
    – Eddymage
    Jun 9, 2022 at 10:09
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    \$\begingroup\$ @Eddymage That’s general, I’ll add some notes about the lion specifically later today. \$\endgroup\$ Jun 9, 2022 at 10:13
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    \$\begingroup\$ @GroodytheHobgoblin It looks like +1 CR for CRs 1-3 ish, +2 CR for CRs 4-10 ish, and +3 CR for CRs 10+. To be clear, these are just rough estimates, since CR is already a rough estimate. \$\endgroup\$ Jun 9, 2022 at 14:27
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    \$\begingroup\$ Thanks for the explanation. It is kind of weird: granted, 5e makes no assumptions about access to magic weapons, but in nearly any campaign I played in, the non-caster players get really pissed if they do not at least have a +1 weapon by around level 10. Maybe it's just our group's expectations. For us, this means resistance does actually LESS to make the monster more dangerous at those high CRs where the DMG math tells you it does more. \$\endgroup\$ Jun 9, 2022 at 14:30
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    \$\begingroup\$ I'm accepting this answer because of the more concrete quantitative discussion (and DMG references, which I totally overlooked when I tried to answer this question on my own - thanks!). However (and as you point out in your answer) @Philipp 's answer deserves notice as well for pointing out that some monster features (resistance in particular) can be virtually ignored in the challenge calculus if the party can reliably negate or bypass them. \$\endgroup\$
    – Izzy
    Jun 9, 2022 at 16:24
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This is a good example which shows how trying to do encounter design by looking at only the challenge rating is a flawed approach.

As the answer by Thomas Markov said, the DMG recommends that resistance should increase challenge rating by factor 1.5. But that's because the DMG does not know if your players have ways to overcome the resistance. The truth is:

If the players don't have a way to deal magical damage: The encounter is twice as hard, because the creature survives twice as long, allowing it to deal twice as much damage over its lifetime.

If the player's primary ways of dealing damage are all magical: Encounter difficulty doesn't change at all, because whether or not it has resistance doesn't change the outcome of the encounter.

The DMG seems to assume a 50% probability that any damage dealt by the players matches the resistance, so it eyeballs factor 1.5 as a compromise. But it can not account for whether or not the combatants have ways to exploit each others weaknesses or mitigate each others strengths. So when you design encounters for your players, always consider how the properties of the enemies match up with what their characters can and can not do.

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  • \$\begingroup\$ "trying to do encounter design by looking at only the challenge rating is a flawed approach." Absolutely true. I use challenge rating to "sketch" the encounter, and then fill in the details and round the edges to fit my party. I certainly haven't memorized the entire DMG, so it's challenging (pun intended) to design appropriate challenges wholecloth without detouring into CR-land. \$\endgroup\$
    – Izzy
    Jun 9, 2022 at 16:20
  • \$\begingroup\$ "seems to assume a 50% probability that any damage dealt by the players matches the resistance" I wouldn't put it exactly like this: If the party consisted of 1 Wizard casting Fireball every round with the dex save failed, dealing 28 points of damage and 14 fighters all of which hit in unarmed combat for 2 damage each, this would mean 50% of the damage damage was magical while the chance of the damage being magical would be ca. 6.7%... \$\endgroup\$
    – fabian
    Jun 9, 2022 at 18:56
  • \$\begingroup\$ Also your maths doesn't seem to check out: 50% of the damage being affected by resistance would mean the damage compared to the damage without resistance D becomes 0.5 * D + 0.5 * 0.5 * D = 0.75 * D, i.e. the fight would take 4/3 as long, not 3/2 as long. The correct factor of the damage overcoming the resistance should be 1/3 which makes the calculation of the damage with resistance D * 1/3 + D * (2/3) * (1/2) = D * 2/3 i.e. any creature would survive 3/2=1.5 times as long as without resistance. \$\endgroup\$
    – fabian
    Jun 9, 2022 at 19:00

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