The period of time after the mutation occurred, when civilization collapsed.
From: Unto Zeor, Forever, Ch 7. 'There was only one room on this level, the Memorial to the One Billion. Every Householding had been built around such a room, a remembrance of the countless millions who had died during the fall of the civilization of the Ancients, during the ensuing Sime~Gen wars, during the thousand years of chaos and darkness of the human spirit.'
(Source: Index card file. "The Ages of Chaos" --- Unto Zeor, Forever) "An interregnum of about a thousand years between the collapse of civilization due to the sime mutation and the reconstruction around an armed truce, confining all simes to their island Territories."