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 include any magical beings with a primary drive... 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 Step into a 15,000-year history through the eyes of its leas... with no idea what’s in that dark world.