Part of the electromagnetic spectrum only extant in The home of the Darkness. It doesn't have light in the sense... and some of Umbra’s pocket dimensions. It absorbs all “light” (using the term loosely), leaving anyone who relies on light to see in complete darkness, regardless of tools, bulbs, or flame.
People of the Darkness can control it with magic, allowing for some places to have ordinary light and thus allow visitors. However, the rest of the planet is completely covered in it, leaving the majority of those among the The Mythos refers to six of the Seven Peoples of the Earth, ... with no idea what’s in that dark world.