Kievan Rus'
After the Rus' attack on Constantinople in 860, the Byzantine Patriarch Photius sent missionaries north to convert the Rus' and the Slavs. Prince Rastislav of Moravia had requested the Emperor to provide teachers to interpret the holy scriptures, so in 863 the brothers Cyril and Methodius were sent as missionaries, due to their knowledge of the Slavonic language. The Slavs had no written language, so the brothers devised the Glagolitic alphabet, later developed into Cyrillic, and standardized the language of the Slavs, later known as Old Church Slavonic. They translated portions of the Bible and drafted the first Slavic civil code and other documents, and the language and texts spread throughout Slavic territories, including Kievan Rus'. The mission of Cyril and Methodius served both evangelical and diplomatic purposes, spreading Byzantine cultural influence in support of imperial foreign policy. In 867 the Patriarch announced that the Rus' had accepted a bishop, and in 874 he speaks of an "Archbishop of the Rus'."
What did Cyril and Mehodius come up with to help with the issue of the Slaves not having a written language?
devised the Glagolitic alphabet
The Slavs had no written language, so the brothers devised the Glagolitic alphabet, later developed into Cyrillic, and standardized the language of the Slavs, later known as Old Church Slavonic.
Cyril