The Theory
In the World of Darkness, the Warriors of the Light defeat the Guardians of Dark Crystals. They then meet a Warrior of Darkness.
Just as the Warriors of Light had to defend their world from becoming engulfed by darkness, the Warriors of the Dark were forced to defend their own world from being swallowed by the light many years ago.
At the end of Final Fantasy III, the party has to find their dark counterparts so that they can aid them in their battle against the Cloud of Darkness. Each Warrior of the Dark appears after defeating each of the four bosses guarding the Dark Crystals in the World of Darkness: Cerberus, Echidna, Ahriman, and the Two Headed Dragon.
After each battle, one of the Warriors of the Dark tells the Warriors of the Light important information for the upcoming battle with the Cloud of Darkness. Before the final battle, the Warriors of the Dark sacrifice themselves so that the Warriors of the Light can save their own world.
But if there’s
- Warriors of Light and Dark,
- Light and Dark Crystals,
- and a Cloud of Darkness,
Could this mean that there’s a Cloud of Light?
Evidence
Final Fantasy III deals with duality. There are warriors of the Light and Dark. There are Light and Dark Crystals. Also a Cloud of Darkness, who,
appears as a green-skinned female humanoid emerging from a portal of darkness at its waist. It has a red heart-shaped mark on its forehead, and several mouthed tentacles around it.
The Cloud of Darkness name comes from:
A dark cloud or dark nebula is a type of interstellar cloud that is so dense that it obscures the light from the background emission or reflection nebula or background stars.
Artist Yoshitaka Amano and writers Kenji Terada and Hironobu Sakaguchi probably took this natural phenomenon as inspiration. But in nature, we see light nebulas or Emission Nebulas.
So then, where is the Cloud of Light? What does it look like, and what form might it take?
Before the final battle, the Warriors of the Dark sacrifice themselves so that the Warriors of the Light can save their own world.
Perhaps The Cloud of Light is also no more, after defeating at the hands of the Warriors of the Dark?

Refutation
In the World of Darkness, the Light Warriors battle the Guardians of Dark Crystals. Weirdly enough, there’s no Guardians of the Light Crystal to protect the Dark Crystals, only the Light Warriors. Final Fantasy III isn’t a complete duality. If there is a Cloud of Light, along with Guardian of Dark Light, the lore doesn’t mention theme.
Source(s):
Warrior of Darkness | Final Fantasy Wikia
Guardians of the Dark Crystals | Final Fantasy Wikia
Final Fantasy: The 15 Craziest Fan Theories (That Might Be True)