The 4th-century saint known as “the Great” who wrote the monastic rule still followed by Eastern Orthodox monks was Basil of Caesarea.
Basil lived from about 330 to 379 and served as bishop of Caesarea in Cappadocia. Alongside Gregory of Nazianzus and Gregory of Nyssa, he was one of the Cappadocian Fathers and a major defender of Nicene Christianity during disputes over Arian theology.
His monastic writings are known collectively as the Asketikon, including the Greater and Lesser Asketikon. Rather than presenting a rigid daily timetable, Basil’s questions and answers emphasize communal life, prayer, work, obedience, charity, and care for the poor. This approach helped shape Eastern Christian cenobitic monasticism.
Basil’s rule remains foundational in Eastern Orthodox and Eastern Catholic monastic life. He also influenced Western monasticism: Rufinus translated his ascetical writings into Latin, and Basil’s example helped form the wider tradition from which the Rule of Saint Benedict emerged. Pachomius is important for early communal monasticism, but the Eastern rule in this question is associated with Basil.