Answer:
Blister agents, sometimes called vesicants, are chemicals that cause severe and acute irritation to the skin and mucus membranes.
There are innumerable weaponized blister agents, but probably the most well known is mustard gas which was used widely during the first World War. This was a Sulfur mustard but their are also Nitrogen mustards. Although developed during World War 1, the British blister agent Lewisite was never used in action and was rendered obsolete with the development of an antidote.