It is by far the least influential branch of the Church in Fodlan, due to its power being limited by both the Central Church, and the nobility of the Alliance.
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If Edelgard was sided with, those among the Adrestian Empire who refused to abandon the Church of Seiros fled to the Eastern Church for safety in Imperial Year 1180. Five years later, as the Empire invaded the Leicester Alliance and prepared to capture Derdriu, the Eastern Church evacuated to Fhirdiad in the Holy Kingdom of Faerghus.