Simeon I of Bulgaria

Simeon I the Great (Church Slavonic: цѣсар҄ь Сѷмеѡ́нъ А҃ Вели́къ; Bulgarian: цар Симеон I Велики, romanized: Simeon I Veliki [simɛˈɔn ˈpɤrvi vɛˈliki]; Greek: Συμεών Αʹ ὁ Μέγας, romanized: Sumeṓn prôtos ho Mégas) was the ruler of the First Bulgarian Empire from 893 until his death in 927. Simeon's successful campaigns against the Byzantines, Magyars and Serbs led Bulgaria to its greatest territorial expansion ever, making it the most powerful state in contemporary Eastern and Southeast Europe. His reign was also a period of unmatched cultural prosperity and enlightenment later deemed the Golden Age of Bulgarian culture.

During Simeon's rule, Bulgaria spread over a territory between the Aegean, the Adriatic and the Black seas. The newly independent Bulgarian Orthodox Church became the first new patriarchate besides the Pentarchy, and Bulgarian Glagolitic and Cyrillic translations of Christian texts spread all over the Slavic world of the time. It was at the Preslav Literary School in the 890s that the Cyrillic alphabet was developed. Halfway through his reign, Simeon assumed the title of "emperor" (Tsar), having prior to that been styled "prince" (Knyaz).

Source: Wikipedia — https://en.wikipedia.org/wiki/Simeon_I_of_Bulgaria