Mustard gas is a highly corrosive poisonous gas, first weaponized by Imperial Germany during World War I. When the gas contacts human skin, the usual result is a period intense itching followed by third degree chemical burn wounds.
Most usages of mustard gas were outlawed by the Geneva Protocol of 1925 after World War I. Nonetheless, Italy made substantial use of it during its invasion of Ethiopia, and Japan (which was not a Geneva signatory) used it without restrictions during its war with China. However, in the European theater of World War II, the various participants stood by the Geneva Protocol, and mustard gas was used only in a handful of isolated accidents.
Mustard agents are regulated under the 1993 Chemical Weapons Convention (CWC). Three classes of chemicals are monitored under this Convention, with sulfur and nitrogen mustard grouped in Schedule 1, as substances with no use other than chemical warfare.
Mustard Gas in Southern Victory
Germany introduced mustard gas during the the Great War, and its ally the United States quickly followed suit. By the end of the war, both the Entente and the Central Powers had taken to using mustard gas-loaded into artillery shells as a weapon. During the Second Great War, nerve agents were the more common weapon of choice.
Mustard Gas in Worldwar
In 1943, British Prime Minister Winston Churchill used mustard gas--which he had been stockpiling in anticipation of a German invasion of Britain in World War II, a war in which poison gas was banned, to repel the Race's invasion of Britain. Mustard gas devastated unprepared Lizard forces and permanently broke the back of an invasion that had already been faltering in the face of stiff British resistance. From then until the end of the conflict, mustard gas and other poison gases were used by other Tosevite not-empires.