VX is described by the US Centers for Disease Control and Prevention as “the most potent of all nerve agents”, with a large dose leading to loss of consciousness, paralysis and respiratory failure. Its only known use is as a chemical warfare agent.
VX is banned under the UN Chemical Weapons Convention, to which North Korea is not a party.
The nerve agent, also known as ethyl N-2-Diisopropylaminoethyl Methylphosphonothiolate, was developed in the UK in the 1950s.
http://www.zerohedge.com/news/2017-02-23/mystery-poison-used-murder-kim-jong-nam-vx-nerve-agent