Sentence:  It was last seen and heard of by some macgillic-cuddies above a lonely valley of their reeks, <WSD>duskening</WSD> the greylight as it flew, its cry echechohoing among the anfractuosities: pour la dernière fois,' The blackbulled ones, stampeding, drew in their horns, all appailed and much upset, which explaints the guttermilk on their overcoats. 
Meaning of the word marked with <WSD> tags:  To make dusky or obscure.