The Dark Ages followed the coming of the Rifts. Monsterous creatures, coming through the Rifts slaughtering humans released more PPE into the world, further strengthening the Ley Lines and opening more Rifts.
During this time, the mythical continent of Atlantis returned from it's dimensional traveling to its earthly origin. The returning of a continent raised the ocean levels, recreating the coastlines of every continent, destroying coastal cities and killing more people, further adding to the free PPE powering the Ley Lines and Rifts. Also, Atlantis brought the Splugorth slavers to earth who quickly began their conquesting activities, killing and enslaving humans and D-Bees.
The Dark Ages saw the fall of nearly every human civilization, cities, countries and governments were all thrown down. Mankind lost track of time and much information and technology was lost.