Heraclius (c.575–641) was Eastern Roman emperor from 610 to 641. The lagoon city of Eraclea was named after him.
Venice had its own church organisation from the 800s, but the origins go back to before the Lombard invasion, and events in the Eastern Roman Empire.
The list of lagoon settlements before the year 1000, from the “Istoria Veneticorum” by John the Deacon.
This document is the earliest description of the translation of St Mark to Venice in 827–828, and also of early Venice. This is a translation by René Seindal.
The Birthday of Venice of is traditionally on March 25th, 421 CE, but this is far more legend than historical truth.