A chemical weapon.
Originally manufactured by the Theocracy of Gruul, it's commonly used in chemical-based WMDs, and these days is occasionally fielded by the Blood Pack. One notable incident of use was the Scouring of Jera'tia Colony, where a planet was reduced to barren wasteland coated in liquid filth, completely uninhabitable.
BA-442 exists in a gaseous state. Upon exposure to oxygen, the chemical mixture produces a sickly purple gas; upon exposure to organic materials, this gas will begin to reduce the material to a liquid state via an accelerated rate of decomposition. It also carries many of the traits of more conventional nerve gas.
BA-442 is outlawed by the Ad'Thoro Pact and the Citadel Conventions, and most other Terminus States outlaw its acquisition, trafficking, sale, or use. The Nemean Abyss doesn't have this "problem", and BA-442 is in frequent use there.