A HORRID Chemical blister-agent. I think it should be counted among them. Slow, painful pathetic death on contact. It might not kill you, but after a heavy exposure you'd wish it had.
If I recall death typically occurred from the blisters which formed in the lungs burst and the victim drowned.
I had a patient who was exposed to this during WWII, yes WWII, and his lungs were screwed.