When you are prompted to move your right and left rapidly, it means you are in this condition. There are a few ways this will happen. The most common is the floating enemies whose arms look like whips. When they hit you, this count and you will get this through normal progression of the game. Also, sometimes the Darkness Archers will shoot you with Arrows of Darkness which count. Later on there are Darkness Puddles which you can just walk in over and over, but it would be hard not to have this achievement by the time you get to them.