Monster ability, What CR do you think it is.

[Yardiff]

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.

Thing about dragon fear once you save your immune to that dragons fear for 24hrs. This is save every round whether you made the save once already or not.

 

[dave2008]

Thing about dragon fear once you save your immune to that dragons fear for 24hrs. This is save every round whether you made the save once already or not.

But it also has 1/4 the range and a lesser DC. They are similar. Of course that doesn't really help determine CR because one ability does not make CR.

 

[Quickleaf]

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’d look at the level of party the adventure/monster is meant for, and guesstimate such a PC’s Charisma save.

Then I’d figure out the percentage chance of at least one PC being thus charmed. Or possibly the percentage chance of two PCs being thus charmed simultaneously.

Then, if the monster is intended to be encountered solo, I’d calculate the effective #of PCs (in the DMG encounter calculation method OR whatever method your prefer) as 1 (or however many) less.

However if the monster is encountered with other monsters and/or highly interactive terrain/puzzle, I wouldn’t have the ability alter its CR whatsoever.