I'm interested in adding exhaustion effects to monsters. I might homebrew changes to the exhaustion mechanic, but for now, I'm interested in assessing how dangerous it is if monsters can cause RAW exhaustion.
There is this question about causing exhaustion: Are there any ways to force levels of exhaustion onto another creature?
There is question about removing it: Ways to remove exhaustion
And this question about making a spell causing it: What level should this exhaustion-causing spell be?
When looking at exhaustion (mostly from a combat perspective), the first two levels of exhaustion seem rather benign. The 3rd and 4th have a high impact on combat performance. By 5th level one is basically done for.
The linked question above about removing exhaustion shows that this is a hassle.
There are apparently no official monsters with such an effect. Though, there is the accursed defiler from Kobold Press's Tome of Beasts which is not official but at least playtested.
How can I account for an exhaustion effect when balancing a monster (e.g. in determining CR)? Assume a monster with one attack per turn which after dealing damage asks for a saving throw to avoid one level of exhaustion.
I assume this would have a high impact, so I would also consider adding it as an x/day attack instead or capping the exhaustion stacking at level 3 or 4.
If you have good reasons to say this is a bad idea, please tell me as well.