Sentence:  Then they passed out from the Forum, forced their way through the crowded streets, and soon were through the "Porta Ratumena", outside the walls, and struck out across the Campus Martius, upon the Via Flaminia. It was rapidly <WSD>darkening</WSD>. 
Meaning of the word marked with <WSD> tags:  To get dark (referring to the sky, either in the evening or as a result of cloud).