Long Night

(-1776 to 0) The period of interstellar decline and anarchy between the fall of the Rule of Man (also known as the Second Imperium) and the establishment of the Third Imperium by Cleon I.

It was a period where worlds were cut off from one another, technology was lost and the population on many worlds simply failed to survive. The galaxy is only just emerging from this period which is commonly held to be from -1776 to 0 according to the Imperial calendar.