Z-67

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 later 20th century. The Doctor, who 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)