Lewisite was one of several arsenic compounds prepared as chemical warfare agents during WWI.  It had been developed in the United States towards the end of the war and, in testing, was deemed so potent that it became known as ‘the Dew of Death’. Plans were made to spray the chemical from aeroplanes over German cities.  Fortunately, the war ended before the plan could be put into effect.

  2-chlorovinydichlorolarsine