which battle or event marked the fall of this empire?
A six-year-long civil war followed by attack from Sultan Mehmed's army
Byzantine Empire
A six-year-long civil war devastated the empire, allowing the Serbian ruler Stefan Dušan () to overrun most of the Empire's remaining territory and establish a Serbian Empire. In 1354, an earthquake at Gallipoli devastated the fort, allowing the Ottomans (who were hired as mercenaries during the civil war by John VI Kantakouzenos) to establish themselves in Europe. By the time the Byzantine civil wars had ended, the Ottomans had defeated the Serbians and subjugated them as vassals. Following the Battle of Kosovo, much of the Balkans became dominated by the Ottomans.

The Byzantine emperors appealed to the West for help, but the Pope would only consider sending aid in return for a reunion of the Eastern Orthodox Church with the See of Rome. Church unity was considered, and occasionally accomplished by imperial decree, but the Orthodox citizenry and clergy intensely resented the authority of Rome and the Latin Rite. Some Western troops arrived to bolster the Christian defence of Constant
when was the byzantine empire born what was it originally called 5th century AD and was called Eastern Roman Empire, or Byzantium and when did it fall? 1453
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Byzantine Empire [SEP] Rise of the Ottomans and fall of Constantinople
The situation became worse for Byzantium during the civil wars after Andronikos III died. A six-year-long civil war devastated the empire, allowing the Serbian ruler Stefan Dušan () to overrun most of the Empire's remaining territory and establish a Serbian Empire. In 1354, an earthquake at Gallipoli devastated the fort, allowing the Ottomans (who were hired as mercenaries during the civil war by John VI Kantakouzenos) to establish themselves in Europe. By the time the Byzantine civil wars had ended, the Ottomans had defeated the Serbians and subjugated them as vassals. Following the Battle of Kosovo, much of the Balkans became dominated by the Ottomans.