 He and his brother, Saint Methodius, were sent by the Byzantine Emperor Michael III in 863 to Great Moravia to spread Christianity among the West Slavs in the area.The brothers decided to translate liturgical books into the contemporary Slavic language understandable to the general population (now known as Old Church Slavonic).As the words of that language could not be easily written by using either the Greek or Latin alphabets, Cyril decided to invent a new script, Glagolitic, which he based on the local dialect of the Slavic tribes from the Byzantine theme of Thessalonica.