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DMG p. 277 provides a table and some instructions for the computation of effective hit points based on resistances and immunities when computing the defensive Challenge Rating of a monster, or figuring out how many actual HP a monster should have based on its defensive CR for that matter. However, the example provided in the DMG does not account for a monster (like a mummy) that has both resistances and immunities (to different damage types, of course).

So: if a monster has both resistances and immunities to damage, do you use the greater multiplier or do you apply both multipliers to get your EHP value during the CR calculation?

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If it has both Immunity and Resistance, use Immunity only

You are meant to apply the multiplier for immunity only, in the case that the monster has both immunity and resistance. I suppose it's because immunity is stronger than resistance, and that's the one that goes.


Case Study: The Ice Devil

The Ice Devil has an overall CR 14, has immunity to 3 damage types, and resistance to 3 damage types. By dismantling this monster's design, we can learn how the designers did it and how we can follow that process.

Calculating its Offensive CR

The following is the creature's ability to deal the most damage:

  • Wall of Ice (70 damage): 35 cold damage, hitting at least 2 creatures;

  • Multiattack (64 damage): One Bite (12 piercing + 10 cold), One Claw (10 slashing + 10 cold), One Tail (12 bludgeoning + 10 cold);

  • Multiattack (64 damage): One Bite (12 piercing + 10 cold), One Claw (10 slashing + 10 cold), One Tail (12 bludgeoning + 10 cold).

The Ice Devil's DPR is \$\frac{70+64+64}{3} = 66\$, which puts it at CR 10. At this DPR, it should have a +7 to-hit bonus, but the Ice Devil has a +10. This means its final Offensive CR is CR 11.

Calculating its Defensive CR

So we know the final CR and the Offensive CR, and so we can get the Defensive CR. This is:

\begin{align} \text{Round}(\frac{x + 11}{2}) = 14 \end{align} \begin{align} x = 16,17, \text{or } 18 \end{align}

Now let's start removing points off this CR.

  • Finding its Effective AC: 22

    • It has 4 saving throw proficiencies, so its AC is increased by 2 for the sake of CR calculation. It also has the trait Magic Resistance which increases its AC by another 2 for the sake of CR calculation. Its listed AC is 18, so its EAC is 22.
  • Base Defense CR: 11

    • The listed HP for the Iced Devil is 180. Without multipliers and an EAC of 22, this means its Defensive CR should be CR 11. But it's clearly not.
  • Defense CR, only Resistances: CR 13

    • So let's multiply its HP by 1.25 to account for its resistance to three damage types, and the expected CR is between 11-16 (because its CR is 14). This gets its EHP to 225, with an EAC of 22. Its defensive CR should be CR 13. But again, it's not.
  • Defensive CR, both Resistance & Immunity: CR 19

    • So finally, let's multiply its HP by 1.5 to account for its three immunities to damage types. This brings its EHP to 337.5, and with a EAC 22, its CR should be 19. But this is too much -- it only has a Defensive CR 17, and CR 18 at most. A Defensive CR 19 will put its final CR at CR 15.
  • Defensive CR, only Immunity: CR 16

    • Let's step back, then, and apply only a 1.5 multiplier for immunities, discounting its resistances. Then its EHP is 270. Its EAC is 22, which brings its Defensive CR to CR 16. This is just enough!

Conclusion

Based on the Iced Devil, you only take into account the immunity multiplier, when the creature has both immunities and resistances.

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The multipliers do not stack.

To supplement markovchain's breakdown of a monster, there are a few points to address in the text that answer your question. First, the text only refers to one multiplier (DMG 277, emphasis added):

Using the Effective Hit Points Based on Resistances and Immunities table, apply the appropriate multiplier to the monster's hit points to determine its effective hit points for the purpose of gauging its final challenge rating.

You don't always apply the higher multiplier, only the relevant one

Second, you only apply multipliers when the immunity or resistance makes a material difference to the party the monster will be fighting (DMG 277, emphasis added):

If a monster has resistance or immunity to several damage types- especially bludgeoning, piercing, and slashing damage from nonmagical weapons-and not all the characters in the party possess the means to counteract that resistance or immunity, you need to take these defenses into account when comparing your monster's hit points to its expected challenge rating.

This passage is pretty weird, given that one might be designing monsters for an adventure where the party composition is unknown. Still, this means that you only apply relevant multipliers. If a monster is immune to psychic damage and resists nonmagical bludgeoning, but your players never deal psychic damage, then you only use the multiplier for the resistance, not the immunity.

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  • \$\begingroup\$ This is doubly weird when you are designing a monster to deal with multiple, heterogenous parties! (in my case -- I have a mummy priestess who is tangled with her own sentient sword which is also +1 but also may pick a fight with the party depending on how the party's been behaving, and the party may not have magic weapons...) \$\endgroup\$ – Shalvenay May 19 '17 at 4:16
  • \$\begingroup\$ @Shalvenay, I guess it makes some sense, since your priestess would be more difficult for your party than for the sword. The different resulting CRs would be representative of the different difficulties the combatants would experience. \$\endgroup\$ – Icyfire May 19 '17 at 4:26

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