Blister agent
Nitrogen mustards are similar to sulfur mustards, but they have nitrogen in them instead of sulfur.  While some countries stockpiled (gathered a lot of) nitrogen mustards during World War II, nitrogen mustards have never been used as chemical weapons.  In fact, today, they are mainly used to treat medical problems.  A few nitrogen mustards are too poisonous to be used for anything but chemical weapons.  But many others are used as chemotherapy medications to treat cancer.