The French expression 'cul-de-sac' describes what the Baudelaire orphans found when they reached the end of the dark hallway, and like all French expressions, it is most easily understood when you translate each French word into English. The word 'de,' for instance is a very common French world, I would be certain that 'de' means 'of.' The word 'sac' is less common, but I can fairly certain that it means something like 'mysterious circumstances.' And the word 'cul' is such a rare French word that I am forced to guess at its translation, and my guess is that in this case it would mean 'At the end of the dark hallway, the Baudelaire children found an assortment,' so that the expression 'cul-de-sac' here means 'At the end of the dark hallway, the Baudelaire children found an assortment of mysterious circumstances.