Monster ability, What CR do you think it is.

This was from a 3rd party 5e campaign called the Blight.

Ability: Every round any creature is within 30' of the monster they must make a DC 12 CHA save or be 'charmed'.
'Charmed': Wont attack monster. Lasts 1 min or until creature takes damage.

I haven't read the creature so this is just what we encounter.

I wouldn't expect a creature called the blight would have a charm aura.

The campaign is called the Blight. This is just one of the monster from that campaign setting.

Do you have anything more on the creature? I mean usually most creatures with auras are a high CR, but the aura itself doesn't necessarily mean that it is a high CR creature. What level were your characters?

The creature had several other abilities like suggestion and hypnotic pattern and telepathy. In the campaign guild it said its a CR9 monster. The GM was only running some tests with our 6 person 3rd lv party.

Does the charm break if the monster attacks the player?

If so, I wouldn't have this ability affect the CR at all. If not, I'd have it boost the monster's CR by 1 if it was 3 or less without the feature, and consider doing so if the CR was 5 or less without.

But since CR is mostly a measure of how many hit points of damage a monster can dish out in one round and how many hit points of damage it can sustain, I wouldn't think about this for more than a couple seconds, only boosting the CR by 1 at most if your gut feeling is "dang! That's a mean feature."

If we compare it to a dragon's fright aura, this aura is weaker (less range and lower DC) with a similar effect on difficulty when compared to a DC 13 adult white dragon. Without anything more to go on I would say less than CR 13, probably close to CR 10.

Ballpark calculation... if the monster is smart enough to target uncharmed PCs before charm victims, then each time someone fails a save, someone else will have to divert some of their firepower to damaging the person who failed the save, in order to break the charm. I'm going to estimate this has a net cost to the party of 1 PC-turn for each failed save*.

Bards, clerics, paladins, sorcerers, and warlocks are proficient in Cha saves. Three of those classes are also Cha specialists, and will likely have maxed-out Cha saves. The cleric and paladin have some incentive to invest in Cha, but rarely specialize. Everyone else is making do with a flat Cha bonus. After some back-of-the-envelope calculations, I estimate around a 60% chance to make a DC 12 Cha save (will be slightly higher for a high-level party, lower for lowbies, but that's the general ballpark).

So: If the party's offensive firepower is reduced to 60% of normal, then the monster's effective hit points are multiplied by (1 / 0.6) = 167%. So, factor that into its defensive CR. Typically it's going to be +2 to +3 CR based on the DMG guidelines, but there's a fair bit of flex in that.

This does, however, assume that the party understands how the monster's ability works, and adopts a policy of whacking each other to break the charm, and have the means to do so without causing too much pain. If they just keep focusing fire on the monster itself, then the effect is far more dramatic. Very quickly, the entire party will be charmed except for the monster's current target, and the combat will turn into a series of duels as the monster takes each PC one-on-one.

[SIZE=-2]*There are a lot of variables here which could cause it to be more or less than that. A dual-wielding fighter with 3 attacks could break the charm by diverting just one attack. But that attack might miss. And if the charmed PC's turn comes right after the monster's, that's another turn of not attacking. And the charmed PC is taking damage, boosting the monster's firepower. I really don't know how it all shakes out, so I'm gonna assume it's a wash and call it 1 PC-turn.[/SIZE]

As far as I know the charm breaks when the creature charmed takes any damage.

so, now I've consulted the DMG's chart on how the features in the monster manual might affect CR, and found 2 items that I'd compare this too, neither of them a perfect fit, but I'd suggest the second is better:

1)The vampire's charm (because it's a charm! but it only targets one foe) has no effect on CR.
2) a dragon's frightful presence (because it has the same result - the affected PCs can't damage the monster) has an interesting effect on CR: "Increase the monster's effective hit points by 25% if the monster is meant to face characters of 10th level or lower."