I was re-reading A Thousand Years of Nonlinear History and this is what happened.
Societies, being constructed primarily of energy and controls for energy flow, have a shadow form - The Inverse Energy Empire, vampire societies. Everything is stolen and pirated, hence the archaic and dated forms, all aping the host kingdom ~200 years prior. In the younger societies, these vampires are the barbarians, and can usher in the night of civilisation easiest, as are hosts weakened by plague and war - finishing off the host in this state ensures a swift, sharp end to the vampire mirror. An exhausted man succumbing to lingering illness.
Corrupted, twisted dated smoky mirror forms, a draining, a great void of energy attracting the societal output through the slow medium of history. As they change the host they change themselves in the future, a nonlinear feedback loop, the vampires plotting their future selves through past historical crimes and subtle manipulations, each a master of psychohistory.