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Z-67, also known as Sullivan's gas, was a nerve gas that unravelled the DNA of Zygons, essentially turning them inside-out. If released it would create a chain reaction in the atmosphere, such as that a single use would kill all Zygons on an entire planet.
Z-67 was developed by UNIT naval surgeon Harry Sullivan at Porton Down, working on Zygon prisoners captured from their attempted invasion in the 1970s/80s. The Doctor, whose twelfth incarnation later called Sullivan an "imbecile" for creating it, confiscated the gas and the formula from UNIT.
The red Osgood Box supposedly contained Z-67 that would be released if the humans chose the "winning" button. However, this was a feint by the Twelfth Doctor, and the box was empty. (TV: The Zygon Invasion, The Zygon Inversion)