From the quest guide:
... On a (surge) result, he places I fatigue token in his play area. The overlord cannot do this if Raythen performs an action to
reveal an objective token
What does the underlined this refer to?
The option to discard an OL card to upgrade a yellow die to a green or the ability to roll a die at all? It is somewhat ambiguous, but I think it refers to rolling a die at all.
If you would have quoted the sentence before that it would be more than clear.
I only have the German quest book but it says in the sentence beforehand something like:
If the heroes reveal an objective token, the overlord may roll a yellow die.
So yeah, he can not do that if Raythen performs an action to reveal an objective token.