Astra decided she was sick of Constantine and made a deal to speed up his death, which was 10 years in the future at that point due to lung cancer.
Constantine is dying of cancer now thanks to Astra, and ended the hour coughing up blood.
What do the DC comics tell us about his fate?
Most of Legends of Tomorrow’s aptly-named “A Head of Her Time” revolved around the beheaded Marie Antoinette who returned to her time to use a magical perfume to charm people to death, but the secondary story revolving around John Constantine is the one that will have lasting consequences.