Cyrillic bears the name of Saint Cyril, but he did not create it; he created Glagolitic. When his and Methodius's disciples, Clement of Ohrid and Naum among them, were driven from Moravia in 885, they were received by Boris of Bulgaria, who had recently accepted Christianity and wanted a Slavic-lettered church free of exclusive dependence on Greek clergy.
In the literary schools of Preslav and Ohrid around the turn of the tenth century, a practical alphabet took shape: the familiar Greek uncial letters for the sounds Greek shared with Slavic, and new letters, several visibly adapted from Glagolitic, for the sounds it did not, the hushing consonants, the yers, and the nasal vowels. The result honoured the missionary founders in its name while being far easier for Greek-trained scribes to learn and write than Glagolitic.
From Bulgaria the alphabet spread with Church Slavonic to Serbia and to Kievan Rus after its conversion in 988, becoming the common script of Orthodox Slavdom. Reforms over the centuries, most notably Peter the Great's civil type of 1708 and the Russian orthographic reform of 1918, trimmed and modernised the letter set into the national alphabets used today. The oldest dated Cyrillic inscriptions come from tenth-century Bulgaria, in the generation after the literary schools were founded.