I am the Cloud of Darkness. What I do not smother, I devour. What I cannot devour, I destroy. The covenant is everlasting. All mortals before the flood of darkness will sink lifeless into the void!The Cloud of Darkness
She appears in the Crystal Tower storyline as a disembodied voice speaking from the World of Darkness and later appears as the final boss of the storyline.
Scholars who study the voidsent have long striven to categorize the fiends by brawn rather than species. The result is popularly known as the twelve-tiered voidsent hierarchy. The voidsent known as the Cloud of Darkness is doubtless in the highest tier of this hierarchy─hence why Emperor Xande entered into a dark covenant with it. In both strength and size, the Cloud is unmatched. A Voidgate of monumental proportions would be necessary for it to enter our realm. Opening such a rift would, in turn, require a power as massive. Enter, then, the Crystal Tower, and the energy it could produce.NOAH Report: Cloud of Darkness
Long ago, the resurrected Emperor Xande of the Allagan Empire forged a pact with the Cloud of Darkness to reduce Eorzea into nothingness, having realized one's achievements will be meaningless upon death. Though Xande died at the hands of the Warriors of Light, the pact remained. Doga and Unei attempt to negate it and close the portal leading to the World of Darkness, but the Cloud of Darkness resists and summons Doga and Unei Clones to attack the NOAH group, pulling Doga and Unei into its world.
As the Cloud of Darkness cannot harm Doga and Unei directly as they possess Xande's blood, it resolves to use the other voidsent to kill them. Having been inadvertently dragged in by the Cloud of Darkness, Nero tol Scaeva protects the two while allowing the void to enter his body. The Warrior of Light and G'raha Tia find the Cloud of Darkness and reduce it into a formless cloud as it makes a final attempt to stop the Warrior's group from escaping the World of Darkness. The Cloud learns G'raha possesses Xande's blood, and is powerless to harm him. Doga and Unei remain to fend the entity off as G'raha returns to Eorzea to end Xande's covenant with the Cloud.
During the Warrior's adventures in the First, they help Ryne bring life back to the Empty by fighting versions of the primals from the Warrior's memories to restore the land's elemental aether. When fix elements were not enough as the Aether was being drained, Gaia is influenced by Mitron to suggest applying darkness Aether. Gaia and Ryne work together to create a version of the Cloud of Darkness from the Warrior’s memories to achieve their goal.
The Cloud of Darkness's appearance is based on her counterpart in Final Fantasy III, being a green-skinned female form with yellow tentacles formed from what appears to be a literal "cloud of darkness". A disembodied voice emanates from a Voidgate shaped similar to the NES map sprite of Cloud of Darkness in Final Fantasy III. Despite being considered genderless, the being is referred to as a "she" in the cutscenes and bears an overall feminine form. When summoned through Eden's power in the First, her design is identical to her appearance in Dissidia, and is smaller in size.
The Final Fantasy XIV version of the Cloud of Darkness refers to itself in singular pronouns. As an embodiment of the concept of nothingness, she fixates on destruction and intends to flood Hydaelyn with darkness, as per the covenant.
A Primal version of her, resembling her appearance in Dissidia Final Fantasy NT, appears as boss in Eden's Promise: Umbra with attacks mostly named after her Dissidia moveset and her title also a direct reference.