A great treasure has been stolen from the Palace and taken to the nearby Temple to the Bitch-Goddess Atrath Klee. The King demands that the party go and retrieve it - the only snag is that the Temple is guarded by potent magic that will kill any male who steps inside. It's just as well that the King has a stash of Girdles of Gender Alteration for just this circumstance.
The small coastal town of Her-Desh has vanished - the bay where it once stood is now unspoiled countryside. Moreover, the locals claim to have never heard of the place. Perhaps the Priests of the local Temple can shed some light on the mystery...
Believable magic, like ordinary physicis, operates according to some invariable laws that always result in some kind of cost or "bounce back". The grater the magic, the more it should cost the character physically, emotionally, and spiritually.