Nova 6 was designed as a super weapon by the Nazis. A Nazi who goes by the name of Steiner created a violent bio weapon that could mass murder millions of people in a matter of seconds. What the Nazis planned to do was launch the gas in V-12 missiles from Siberia onto locations such as Washington and Moscow. However the weapon was designed too late in the war and when the Nazi regime began crumbling, Hitler asked for the weapon to be destroyed so it couldn't be used by anyone else.
Steiner disobeyed this order and remained with the last of the Nazis to protect the weapon. Steiner hating america, he made a truce with the soviet regime to give them the weapon in exchange for protection. The last of the gas was stored in a shipwrecked ship and Steiner leads the Russians to it. Dragovich, the russian superior, decides to lock his own men in a row to test the weapon on them. Among these men, Dimitri Petrenko, one of the main characters from Call of Duty: World At War, made another appearance in Call of Duty: Black Ops as a supporting character, he is killed. This pisses off Reznov who destroys the ship. Yet Steiner escapes which allows more samples of the gas to be created.
The gas is produced on Rebirth island. Rebirth island is also one of the few places where the gas is unleashed and changes the gameplay. At one point in the game, the player wears a hazmat suit to stay alive in the gas. He can only take a limited amount of shots before the suit cracks, killing him. The game insinuates that the gas is tested on monkeys and the scientists who worked on Rebirth are actually ex-Nazis. It's never made clear what the gas is exactly composed of, just that it's deadly.
Dragovich planned to use the gas on many cities such as New York, L.A., Boston, Atlanta... The gas was meant to be activated by dormant Soviet agents who lived in the United States. Activation codes would be transmitted from an underwater base that's attached to the bottom of the Russian ship the Rusaka. The codes would be transmitted to the brainwashed agents who would then go find the bomb, thanks to the numbers, and activate it, once again thanks to the Numbers.
When exposed to Nova-6, the victim is usually dead before they hit the floor. The yellow gas causes violent coughing which leads to vomiting. As the victim suffers from extreme pain, their body blackens from necrosis before death occurs.