Little is known with any certainty about the entity called Lux. Most who know about Lux—which includes the Yaga, Abnaxus, The Mole and the Oular—consider Lux to be an important, even vital, part of existence, the First Dreamer, who existed before time, and whose dream created and now sustains the current world.
According to the Yaga, Lux stole their soul-stone so that Lux could use its power to dream the universe into existence. The stone—Lux’s heart, according to Abnaxus—was then itself stolen by Roper Klacks, leading to Lux’s present ill state.
As the avatar of Dreams, Lux exists in balance and opposition to two different forces. The first of these is the Yaga, which represents nightmares; the second is the Undreaming, representing destruction.
By the time Dreamfall Chapters begins, Lux is ill and dying, the Dreamer's resting body guarded by the Oular of the Purple Mountains and cared for by Abnaxus of the Venar. Once the Dreamer, they believe, the dream will cease to exist. In order to forestall that event, Abnaxus attempts to use dreams to contact Zoë Castillo, whom, as a Dreamer herself, is the one person able to save Lux.
Zoë arrives, bringing with her Lux’s soul-stone, retrieved from the Yaga, and with it, the Yaga themselves. Using the soul-stone, Lux—the Dream—and Nox—the Nightmare—come together inside Zoë, granting her their power and the responsibility of remembering them and dreaming the Dream until it is time to pass it on.
Lux appears once again after Zoë faces the Undreaming, currently inhabiting Brian Westhouse. Together, Lux and Zoë, the two Dreamers, confront the Undreaming directly and, convincing the Undreaming that they understand that one entity cannot exist without the other—creation requires destruction—accept the Undreaming as part of themselves, and restore balance to the Dream.