Of, resembling, or relating to twilight.

She halted at the top of the stairs, stunned by the confusion in the crepuscular gloom of the Weyr Bowl.

Piemur and Farli were motionless in the crepuscular light that briefly heralded the dawn.

So… she’s describing pre-dawn light by saying it resembles twilight.

Not to be confused with corpuscular.

Definition from Kindle