When calculating a monster's challenge rating, an important step is to multiply the monster's effective hit points if the monster has three or more immunities/resistances that will give the players significant trouble. For CR 17+ monsters, having three such resistances has no effect, and having three such immunities applies a 1.25x multiplier. But I haven't seen any CR 17+ monster that uses this multiplier. Does such a monster exist? What immunities would warrant giving a monster this multiplier?
A demilich should, but I haven't verified if its CR matches the DMG math - many monsters do not. A tarrasque 100% should, since B/S/P from a nonmagical attack won't work and neither will damage of any type from any ranged spell attack (or line spell targeting it, for that matter, although that's very rare), meaning without magical weapons (which the game is allegedly balanced around - PCs should never be entitled to magic weapons), all ranged attacks generally bounce off.
Those 2 monsters may be bad examples. The demilich relies on a drop to zero AOE, so it's hard to adjudicate its challenge rating. And I think the tarrasque punches above its challenge rating even without the multiplier.
My point is that I don't know of any examples of monsters that use this multiplier. And that makes sense. It would take an absurd number of immunities to keep even one character of any class from dealing damage at that level. One set of immunities that could theoretically invoke this multiplier is something like "B/P/S from Metal Weapons or Nonmagical Attacks". By the late game, the martial classes should all have access to magical attacks, but with Nonmetal Weapons? That may be enough of a hinderance.
My point is that I don't know of any examples of monsters that use this multiplier. And that makes sense. It would take an absurd number of immunities to keep even one character of any class from dealing damage at that level. One set of immunities that could theoretically invoke this multiplier is something like "B/P/S from Metal Weapons or Nonmagical Attacks". By the late game, the martial classes should all have access to magical attacks, but with Nonmetal Weapons? That may be enough of a hinderance.
It isn't supposed to completely prevent all damage options, just make them have to use suboptimal options or trial and error their way through. If the immunity catches even a single attack before they switch damage types or forces them to use options that deal even 1% less damage, it effectively increased its HP by that amount. That is all it means.
Those 2 monsters may be bad examples. The demilich relies on a drop to zero AOE, so it's hard to adjudicate its challenge rating. And I think the tarrasque punches above its challenge rating even without the multiplier.
Just went to work on the Tarrasque, and it's DCR 34 if you ignore its damage immunities and reflective carapace. With them interpreted as the immunities buff, CR math becomes impossible, because the RAW on calculating CR means that a creature with effective HP of 851 or more causes the rules to explode - you have to calculate CR by looking it up in the table, and the Tarrasque's effective HP becomes 935. If you extend the table - the algorithm is easy to predict - it is in the HP band for DCR 32, giving it net DCR 36, as the AC calculation doesn't change (it is effective AC 30: 25 + Magic Resistance + 3 Save Proficiencies + Constrict).
The OCR on a Tarrasque is 31, if my math is right - DPR rounds to 279, attack bonus +19..
So Tarrasques are a monster whose CR doesn't match the DMG math - it's CR 33 before its damage immunities and reflective carapace, CR 34 if those count as "damage immunities" for the multiplier. That's normal and expected, as I mentioned - many monsters have a CR that doesn't match the DMG math.
Bear in mind, Defensive CR is a joke, because condition immunities are ignored entirely, and worse, saves contribute to DCR based purely on how many the creature is proficient in, ignoring its actual bonuses (traits that improve saves apply separately). A tarrasque's save line is 10/0/10/5/9/9, but its DCR only goes up due to the 5/9/9, ignoring the 10/0/10. You could bump it to 10/9/10 or 10/0/19 without its DCR going up at all.
When calculating a monster's challenge rating, an important step is to multiply the monster's effective hit points if the monster has three or more immunities/resistances that will give the players significant trouble. For CR 17+ monsters, having three such resistances has no effect, and having three such immunities applies a 1.25x multiplier. But I haven't seen any CR 17+ monster that uses this multiplier. Does such a monster exist? What immunities would warrant giving a monster this multiplier?
A demilich should, but I haven't verified if its CR matches the DMG math - many monsters do not. A tarrasque 100% should, since B/S/P from a nonmagical attack won't work and neither will damage of any type from any ranged spell attack (or line spell targeting it, for that matter, although that's very rare), meaning without magical weapons (which the game is allegedly balanced around - PCs should never be entitled to magic weapons), all ranged attacks generally bounce off.
Those 2 monsters may be bad examples. The demilich relies on a drop to zero AOE, so it's hard to adjudicate its challenge rating. And I think the tarrasque punches above its challenge rating even without the multiplier.
Quindraco still pointed out that many official monsters do not follow the CR formula they give us for homebrew.
Still if you do have an example of what you are talking about, that would be helpful.
My point is that I don't know of any examples of monsters that use this multiplier. And that makes sense. It would take an absurd number of immunities to keep even one character of any class from dealing damage at that level. One set of immunities that could theoretically invoke this multiplier is something like "B/P/S from Metal Weapons or Nonmagical Attacks". By the late game, the martial classes should all have access to magical attacks, but with Nonmetal Weapons? That may be enough of a hinderance.
It isn't supposed to completely prevent all damage options, just make them have to use suboptimal options or trial and error their way through. If the immunity catches even a single attack before they switch damage types or forces them to use options that deal even 1% less damage, it effectively increased its HP by that amount. That is all it means.
Just went to work on the Tarrasque, and it's DCR 34 if you ignore its damage immunities and reflective carapace. With them interpreted as the immunities buff, CR math becomes impossible, because the RAW on calculating CR means that a creature with effective HP of 851 or more causes the rules to explode - you have to calculate CR by looking it up in the table, and the Tarrasque's effective HP becomes 935. If you extend the table - the algorithm is easy to predict - it is in the HP band for DCR 32, giving it net DCR 36, as the AC calculation doesn't change (it is effective AC 30: 25 + Magic Resistance + 3 Save Proficiencies + Constrict).
The OCR on a Tarrasque is 31, if my math is right - DPR rounds to 279, attack bonus +19..
So Tarrasques are a monster whose CR doesn't match the DMG math - it's CR 33 before its damage immunities and reflective carapace, CR 34 if those count as "damage immunities" for the multiplier. That's normal and expected, as I mentioned - many monsters have a CR that doesn't match the DMG math.
Bear in mind, Defensive CR is a joke, because condition immunities are ignored entirely, and worse, saves contribute to DCR based purely on how many the creature is proficient in, ignoring its actual bonuses (traits that improve saves apply separately). A tarrasque's save line is 10/0/10/5/9/9, but its DCR only goes up due to the 5/9/9, ignoring the 10/0/10. You could bump it to 10/9/10 or 10/0/19 without its DCR going up at all.