Complex sentence: "To descend into the cave of Trophonios" became a proverbial way of saying "to suffer a great fright": this saying is alluded to in Aristophanes's "Clouds".
Split into simpler sentences: To descend into the cave of Trophonios became a proverbial way of saying "to suffer a great fright". This saying is alluded to in Aristophanes's "Clouds".