The Dark Ages is the period of roughly a thousand to twelve hundred years after the Purge.
During that period the magical spells and artifacts the Paragons used to keep the world tranquil and hospitable ceased to work or started to malfunction. This caused natural disasters that ripped continents apart, caused the nature to grow wild and untamed and opened previously sealed planar rifts.
Previously tame or docile monsters grew aggressive and wild. Fiends from the lower planes and fey from Feywyld marauded through the world spreading corruption and chaos. Most of the mortal civilizations fell, either due to external threats or internal strife. Mortal tyrants used the chaos to usurp power, and established new brutal empires.