Every year, on a particular night, citizens of the Old World shut their doors and windows, snuff out their hearths and quiver in fear, huddled together, and pray for the dawn. This night is Geheimnisnacht - Night of Mysteries - when the dreaded Chaos Moon, Morrslieb, is full and closer than ever to the world whilst Mannslieb also reigns in the sky.
During this Night of Mysteries mutations spring up, Chaos abounds, and Necromancers roam the world reaping their deadly tolls. It is a universally dreaded time among the sane, and the holiest of holies for those with a dark soul.
Geheimnisnacht is only the Reikspiel name for it, however, as each culture on the planet recognises its importance and holds it as a dark time above all others. Winter’s Eve, Ar’Uzkul, Twilight’s Tide, and possibly the Dark Elves’ “Death Night” (in which Witch Elves scourge their cities for sacrifices to their evil god of murder, Khaine).
With the proximity of so much warpstone, it is not surprising that the eerie glow of the Chaos Moon causes the just to hide whilst the bestial roam free in an ecstasy of worship for their vile gods…