I was out and about that night, taking my trench coat for a walk, when a sudden fog came rolling down the street towards me like a grim grey tidal wave. I stopped, and studied its progress cautiously. We don’t get many fogs, in the Nightside. We get lots of rain, and thunder-storms, and the occasional hail of frogs, but we don’t really do weather, as such. Weather and seasons are part of the natural order of the world, and we don’t really do natural either. So a sudden fog always means trouble for someone.

People on the street were already running ahead of the fog, or disappearing into sheltering doorways, as the thick pearl grey wall rolled relentlessly on, enveloping clubs and shops and soaking up the neon light, till only the merest Technicolor glints showed through, like so many half-blinded eyes. A growing silence moved with the fog as it ate up all the life and laughter in the street. I could see dim shapes moving, caught in the thick mists, struggling slowly like insects caught in hardening amber. The fog smeared itself across shop-windows, filling the night and hanging heavily on the air, surging forward in sudden, billowing clouds. Up close, the pearly grey mists were full of shimmering sparks and uncertain shapes that came and went in a moment. I seriously considered running.

This had all the makings of a flux fog.

Such things are dangerous. A flux fog means the corners of the world aren’t properly nailed down any more, and reality is up for grabs. Inside a flux fog, all certainties are thrown into question, and all the possibilities that ever were are suddenly made equal. Take the wrong turning, in a grey world where every turn looks just like every other, and you could end up walking out of the fog into a whole new place. With no guarantee you’ll ever find a way home again. Everything looks blurred and out of focus in a flux fog because you’re seeing a dozen different dimensions, a hundred possibilities, for every object or person or direction. People and places can change subtly even as you approach them; familiar faces can become strangers, and in the blink of an eye you’re trapped in a world that never knew you. The only real defence against a flux fog is not to be there when one manifests.