Angel sees it in a few peoples' minds, describing it as an "incoherent orange light" that was pretty much terrorizing them. It may have something to do with how many people in Mark's underground facility had repetitive "be perfect be perfect be perfect be perfect..." thoughts.
It may be able to spread its influence through rallies and various visual/auditory media. Young girls like Beth are also seemingly capable of doing so. One of Mark's subordinates, Toni, may be one of such girls, but she is equally as terrorized by the One Light.
Throughout Maximum Ride Forever, many brainwashed Doomsday Group members appear saying that they serve the One Light and some groups chant the phrase over and over; however, they also serve the Remedy. The One Light's true nature remains unknown.