Czech language
Around the sixth century AD, a tribe of Slavs arrived in a portion of Central Europe. According to legend they were led by a hero named Cech, from whom the word "Czech" derives. The ninth century brought the state of Great Moravia, whose first ruler (Rastislav of Moravia) invited Byzantine ruler Michael III to send missionaries in an attempt to reduce the influence of East Francia on religious and political life in his country. These missionaries, Constantine and Methodius, helped to convert the Czechs from traditional Slavic paganism to Christianity and established a church system. They also brought the Glagolitic alphabet to the West Slavs, whose language was previously unwritten. This language, later known as Proto-Czech, was beginning to separate from its fellow West Slavic hatchlings Proto-Slovak, Proto-Polish and Proto-Sorbian. Among other features, Proto-Czech was marked by its ephemeral use of the voiced velar fricative consonant (//) and consistent stress on the first syllable.

Q: What alphabet did the missionaries bring to the West Slavs?
Glagolitic