The period of time with no Emperor or Empire, between Adron's Disaster and the restoration of the Empire by Zerika the Fourth. It lasted about 250 years (497 years by Verra's reckoning). The Empire stopped in the reign of a Phoenix, and was restored by another Phoenix.
It is marked by the loss of the Imperial Orb which made the use of Sorcery impossible for all but the most powerful wizards.
During this period, there was a total breakdown in the normal structure or Dragaeran society, which dramatically affected trade, military engagements, medicine, art, farming, and every other important aspect of Dragaeran life.
These breakdowns were responsible for large amounts of famine, plagues, an increase in crime, and other major problems within Dragaera.
The loss of the orb also allowed armies from the Eastern Kingdoms to invade portions of the Empire. After the end of the Interregnum, these lands were reclaimed.
The nullification of sorcery also had effects on Dragaeran geography. All of the existing floating castles came crashing down as the spells that held them in place suddenly stopped functioning.
Kieron's Watch, a historic rise that jutted into the sea near Adrilankha suffered a similar fate.