The Cloud is a lethal prewar chemical weapon developed before the Great War by Big MT, within the organizations Z- Innovative Toxins Plant. It was secretly tested at the Sierra Madre Resort and Casino, unknown to the staff or guests. After the Second Battle of Hoover Dam, samples were recovered from the Sierra Madre by NCR researchers for purpose as a weapon of mass destruction against the Rupublic's enemies, but by unknown means was unwittingly release over the grater portion of the Core Region and Mojave Wasteland.
Sierra Madre Cloud (Presumably SM-0) Edit
By 2281 the Cloud released from the Sierra Madre's ventilation systems had engulfed the Casino the the point of blocking out the sun. An unknown chemical compound smelling and tasting of copper, sulfur, and plastic, it killed many of the Casino's inhabitants and transformed those in hazmat suits unto the ghost people. Whatever it's nature, the Cloud is a corrosive compound, slowly deteriorating both organic and inorganic matter, which makes gas masks, environment suits, and power armor ineffective protection. The Ghost People were Sierra Madre staff trapped in their hazmat suit when the Cloud eroded the helmet clasps. Slow exposure to the toxin mutated them to an unknown state, fusing their bodies into their suits and forcing an adaptation into a entity composed of pockets of pressurized gas, glowing green fluid, and tissue with a universally connected biological system, that seemingly draws nourishment from the Cloud.
A variant of the Cloud designed by NCR scientists in the early 2280's, SM-5 is a more potent alteration of the Sierra Madre toxin. It kills its victims and corrodes more quickly than the SM strain, as a result none of the Sierra Madre Ghost People are encountered in the Mojave. Rather those who survive in hazmat suits become creatures analagous to feral ghouls, albiet with a higher degree of intelligence.