Void Millennium

The Void Millennium was a period of the Original Universe's history that virtually no-one can remember apart from Deathsaurus.

There are some who claim that the Void Millennium didn't exist, because if it did, someone would be around to remember it. In actuality, it's not that no one can remember it — It's that those who do are, generally, either in the Galactic Government or Space Marines or dead.