There is a tradition in fiction at least of Hexensnacht being a time when chaos powers whisper etc.

What do you think of ruling that someone making any important roll on that night, including recovery and disease recovery checks, and rolling a Chaos Star might get a whispered voice offering them aid, automatic success, recovery etc if they "accept my/our/your blessing".

The blessing would be a big dose of corruption points (4) or what...?

Refusing the offer would invoke a "curse" of some sort, the Ill-Fortuned condition for a month perhaps?

Not sure about this but trying to figure out how to reflect this in the game, including the capricious and unfair nature of Chaos' attentions but still reflecting game balance because purely arbitrary stuff is not really kosher in an RPG (in my view at least).

The above sounds good.

You could allow a check to resist corruption to lessen the amount of corruption gained. That way a high toughness/resilience charcter would not be "cheated" out of his/her roll. The same could be done for the curse, allow the character to roll to decrease the duration of the curse (but maybe use discipline instead).

You could also make the gifts/curses different depending on which chaos god helps. It might make it more varied if several PCs roll chaos stars during the night.

Hmm, like it.

Saying yes, gets 4d Corruption check with one auto chaos star in the pool (so at least one point of corruption being gained).

Saying no, perhaps the curse is a misfortune die but in a field dictated by the involved power: Nurgle - all Toughness checks; Khorne - all Str checks; Slaanesh all Fel checks; Tzeentch all Int checks, with actions where the misfortune die applies also gaining a new outcome line Chaos Star - Chaos Mocks +1 Stress.

Nice idea. Adds some extra depth to the world for the players and gets them thinking upon the consequences of using magic.

As most of our games take place within Bretonnia, the mischievous Faye of the forests, and the influence of the lady of the lake would also gain in power,

I was thinking, there are two sides to chaos, the gods of order may also whisper to there faithful and or actively oppose evil characters actions. Clerics and druidic types may gain blessings, evil deeds by cultists etc may be hampered or opposed.

Thanks.

Just need to figure out duration of the condition. Is it lingering until next Hexensnacht, dependent on having corruption points, or something else?

You could let the PCs roll to shrug off the condition say once a month or week. Could be a discipline or resilience or even that they need to succeed with both a dicipline and resiliance (at the same time?). A high number of corruption points could add difficulty (or black dice) to the check(s), making it harder.

Hmm, a check based on your corruption points, I like that!

With the offer to shrug if off freely in exchange for more perhaps!

We actually moved past this as all my PC's avoided having diseases or issues to roll that night, but of course it will come back around someday.

I did inflict a different sort of curse on one PC just because when back-to-back a PC twice rolls double chaos stars while poking at an ancient Nekharan construction something has to happen!