A brothel is a house of ill repute, where 'tarts' ply their trade. When the gossiping women are talking, one of them mentions the password to get into the brothel, talking about things her husband doesn't think she knows. You should have a speech option with Lepine about it, in any case.
The holy grail is in a cave in the western mountain range in the dark forest, towards the south (a bit north and then west of the imp's deserted castle).
