Blackfog

From publisher blurb: Transport your game to the era of Gaslight! What is Blackfog? Blackfog is perhaps the defining feature of the Fourth Age and the Gaslight era of the Midlands around Lake Argosa; certainly its arrival changed the Third Age world as the inhabitants knew it. However, while it is native to that setting, there is no reason it cannot be imported to any campaign world. It resembles a normal fog but darker, like coal smoke though it is perfectly breathable. It impedes vision and light like a normal fog, but its true danger is what it brings. Blood. Madness. Terror. Rending. Death. It brings Them. Blackfog is a tool for GMS to create uncertainty and horror. It never presents the same challenges twice, keeping PCs a little off-balance. It might even make them afraid of the dark...