The inventor of mustard gas is Fritz Haber, a German scientist. He worked at the Kaiser Wilhelm Institute for Physical Chemistry in Berlin. Haber was a pioneer in the field of gas warfare and developed mustard gas as a chemical weapon. He first used it in World War I against the Allied forces. Mustard gas is a blistering agent that causes severe skin irritation, eye damage, and respiratory problems. It was used extensively in World War I and caused many casualties.