"How art thou fallen from heaven, O Lucifer, son of the morning! (Isaiah 14:12, King James version).

The puzzle is easy enough to disentangle. Lucifer is a fallen angel, and "sons of the morning" here means the band of angels whose role in the Christmas story is to announce the birth of Jesus to the shepherds:

Brightest and best of the sons of the morning,
Dawn on our darkness and lend us Thine aid;
Star of the East, the horizon adorning,
Guide where our infant Redeemer is laid.