Yeah, it’s a sermon on the Dormition (“falling asleep”, aka death) of Mary. What does that have to do with the Feast of the Assumption? Well, the Eastern churches believe that Mary died, was resurrected after three days by Christ, and was then taken up to heaven. The Western churches tend to oscillate between Mary dying first and Mary just getting scooped up like Enoch and Elijah.
St. John of Damascus was a Christian courtier who lived in Damascus — not under the Byzantine Empire, but the Ummayad caliph, in conquered Syria. However, though he had to live as a dhimmi, he didn’t have to live under Emperor Leo the Isaurian, a rabid Iconoclast. From over the border, St. John wrote his influential treatise On Holy Images, saying what his fellow Christians in the Empire could not.
But Emperor Leo apparently got his revenge by have his people forge a letter purporting to be from St. John, which offered to open Damascus’ gates to an invading Imperial army. The Caliph, not thinking about how unlikely it was that St. John would make a deal with somebody who wanted him dead, cut off St. John’s hand. (Some say his hand grew back, which made the Caliph apologize.) But either way, St. John retired to a monastery and continued to write.