The nature of the Melding is still clouded in secrecy. In the CG-trailer the Melding is supposedly the big, looming cloud, but the exact nature of this cloud is still unknown.
As seen in this exclusive intro, the Melding actually came on earth because of Admiral Orestes Nostromo, who took the manual control of the Arclight and crashed it into the city of Fortaleza, Brazil. The cloud was seen near the orbit of Earth, when Arclight used arcfolding.
Walking into The Melding cloud in the game will cause the player's HUD to distort and the environment to darken with a purple hue. Walking far enough will result in light, periodic damage and if the player walks too far, instant death.
The Melding, it says on the Firefall front page, is an "inadvertently" invited "enemy". This seems to imply that the Melding is some sort of lifeform, but this has not been confirmed. In a thread concerning the name of the game, Firefall, it was explained by a Red 5-employee that Firefall was an event preceding the Melding. Thus some think of the Melding as a description of the event where the gigantic cloud arrived on earth, although the term is apparently also used to refer to the cloud itself.
In an audio transmission, the reporter Ines Whitney says "We've been told for years that Melding is a harmless, safe exhaust," which makes it appear that the Melding may have actually been created by using crystite, rather than arriving from somewhere else.
"During my time at Cambridge, I was fortunate enough to work on many projects for Omnidyne, including the in-depth study on the properties of crystite exhaust. It was quite fascinating. The exhaust would, under the right circumstances, absorb and transform microscopic levels of carbon. I came to call the process 'melding' in my notes."