Monophthongization of diphthongs in Proto-Slavic
Monophthongization of diphthongs is a Proto-Slavic sound change in which diphthongs turn into vowels. It is one of the key events in the chronology of the Proto-Slavic language.
Source: Wikipedia — Monophthongization of diphthongs in Proto-Slavic (CC BY-SA 4.0)