The Descending Fog is a challenge game mode in Project Zomboid in which a thick fog coats the map, heavily limiting visibility and giving more opportunity for zombies to sneak up on the player.
In-game challenge description
People first saw it on the water. A faint white blanket over the Ohio river.
It began as a mist. Then it enveloped the town. The townsfolk died screaming in the gloom. Now they walk out there.
Unseen, unheard and relentless in the pursuit of the living.