Complex sentence: Eorzea's history revolves around a series of Umbral and Astral Eras, the former being periods of great calamity--the First Umbral Era marked the end of the age of gods.
Split into simpler sentences: Eorzea's history revolves around a series of Umbral and Astral Eras. Umbral Eras are periods of great calamity--the First Umbral Era marked the end of the age of gods.