For someone not familiar with WHF outside of what this quest has caused me to learn: could you explain? From context I'm guessing a night of supernatural badness that happens once a year.
Geheimnisnacht is the night when Morrslieb (the warpstone moon) waxes full in the sky, this happens twice a year
During this time the veil between worlds gets... thin
During this time rifts grow larger, allowing Daemons to sometimes pierce into reality, the dead grow restless and all manner of dark things (probably Beastmen) stir
The Order aligned races of every stripe buckle down to endure the night while darker forces either run rampant or take the opportunity to further their ends
People of bar their doors or abandon their homes entirely to huddle in more fortified structures
Dwarven Slayers eagerly guard the doors to holds in anticipation of their doom, alongside Ironbreakers
Elves in Ulthuan and Athel Loren alike whisper prayers in the light of sacred flames
Sylvanians light candles for each family member they've lost and place them at the window of the room they were killed in, or at the door if they died away from home
The chaotic and unpredictable nature of Morrslieb's orbit around the world means that Geheimnisnacht occurs at different times each year in unpredictable fashion
And after each one passes the people come out to see which towns got razed to the ground and who disappeared without a trace
They take axes to trees that have been twisted by corruption, burn plagued and rotten crops, and clean out inexplicable gore
Then they give thanks for surviving another Geheimnisnacht and begin counting the days till the next one
Sometimes Morrslieb waxes full at the same time that Mannslieb (the normal moon) does, this is known as The Day of Mystery
The Empire uncharacteristically does not simply bar their doors on the day leading up to this however, instead many throw strange and unique festivals dedicated to Morr
Various fortune tellings, augaries and other such rituals are commonly performed, sometimes with unsettling results
And people commonly take this day to pay homage to their ancestors, it's said that sometimes said spirits even return to commune with the living
The veil between worlds still thin, but it seems fortunately much milder compared to a normal Geheimnisnacht, though it can still be dangerous