How to handle CR of supporting, non-attacking monsters?

[Yukito01]

Hey there.

So, I am planning this fight with the BBEG, but I'm a bit confused about how to handle an aspect of it.

The encounter is the BBEG, three to five minions, and three cultists. Without counting the cultists, the encounter is designed to be hard (the cultists are the problem, see below).

The BBEG is a melee-focused enemy, and so are the minions, but it is being supported from the backlines by the cultists. The three cultist don't directly attack the players; instead, they focus on casting a spell on the BBEG that gives it +2 AC, +10 speed, and a 25% chance of spells failing to harm it, PER cultist. Also, the BBEG recovers 1d6 x the number of cultist HP at the end of each round.

The point of the fight is to force the players to deal with the cultist before engaging the BBEG, right?

My question, how does these kind of non-attacking monsters affect the encounter CR?

In a way, the cultists are just a way to represent extra HP on the monster, without the monster becoming a bullet sponge. But I'm not sure if this approach is the correct one. Maybe I am overestimating their effect, and including a weak attack in their turn would be better? I don't mind if the encounter turns deadly, as it is supposed to be an intense fight.

Anyways, any input on this would be appreciated.

A bit more info on the party: 6 lv 3-4 players, with a heavy focus on melee. No dedicated spellcaster nor healer (they do have some, but not main focus).

[Lunali]

CR is based on the amount of time something will survive and the amount of damage it will do in that time. Use the DMG rules to calculate their defensive CR and halve it to get their overall CR.

[JackPhoenix]

My question, how does these kind of non-attacking monsters affect the encounter CR?

They don't, because there's no such thing as "encounter CR". CR only applies to creatures, nothing else.

[Protolisk]

They don't, because there's no such thing as "encounter CR". CR only applies to creatures, nothing else.

Though true, the CR system is explicitly tied to the XP system, and the XP system is explicitly tied to the encounter building section in the DMG. So although there is no "Encounter CR", there is encounters being Easy, Medium, Hard, or Deadly, which is based on XP values, which are based on CR.

[MaxWilson]

Though true, the CR system is explicitly tied to the XP system, and the XP system is explicitly tied to the encounter building section in the DMG. So although there is no "Encounter CR", there is encounters being Easy, Medium, Hard, or Deadly, which is based on XP values, which are based on CR.

And that is why you don't need to do anything special: the boss monster's XP cost will be 2x normal with three minions, and if anything the encounter difficulty estimate will probably be too high due to this--no need to bump it up further.

[Yukito01]

CR is based on the amount of time something will survive and the amount of damage it will do in that time. Use the DMG rules to calculate their defensive CR and halve it to get their overall CR.

So, even though the cultist themselves pose no danger to the PC, I still should include them in the calculation of encounter difficulty, right? I see. Thanks!

[MaxWilson]

So, even though the cultist themselves pose no danger to the PC, I still should include them in the calculation of encounter difficulty, right? I see. Thanks!

Yes, precisely. They are part of the challenge--they're not noncombatants and they add significantly to the difficulty, so they count.

[Yukito01]

Yes, precisely. They are part of the challenge--they're not noncombatants and they add significantly to the difficulty, so they count.

I understand. Thanks a bunch!