Freya is a deity most commonly worshipped in Caingorn and the Skellige Islands. There is a Temple of Freya on the island of Hindarsfjall.
There is a custom among the more religiously observant royal families whereby any aspiring ruler, prior to assuming the throne, must spend a day among the duchy's simple folk, execute a series of menial tasks, and pray in the Temple of Freya in the evening.
For Deidre Ademeyn, the heir to the Caingorn throne in the module, the problem was that she had displeased the simple folk in the past. And though she most probably could bribe a few prominents, the priestesses of Freya would not accept her. Reportedly Deidre caused a huge wolf to suddenly emerge from behind a house and before it could be stopped, the beast shredded Isildura's throat.
The goddess Freya is only mentioned in this module, there is no mention of Freya in the main game.