What is Novichok, the nerve agent Germany says was used to poison Navalny?

Novichok, a group of nerve agents developed by Soviet Union in the 1970s and 1980s, is said to be 10 times more lethal than nerve agents like VX and sarin. Novichok paralyses muscles, slows the heart, and if the dose is big enough it can lead to death by asphyxiation. Novichok can be inhaled, absorbed through the skin or ingested.