Glagolitic is the first known Slavic alphabet. The creation of the alphabet is generally ascribed to Saints Cyril and Methodius in roughly 862 CE. It was used for religious and governmental writings until the 12th century, when it began to lose out to (the much simpler) Cyrillic alphabet. (Cyrillic, by the way, is named in homage to Saint Cyril, but he did not have anything to do with its creation.)
Pictured here are the Baška tablet, one of the earliest known examples of written Croatian (dating from 1100 CE), and a transliteration of Glagolitic.
this is really pretty-looking