Joker Venom is a chemical formula created by the Joker, which appears to be the substance that disfigured him and drove him insane. While the Joker was incarcerated in Arkham Asylum, Ra's al Ghul approached him with the offer of mutagen in exchange for the formula for Joker Venom. After Shredder and Ra's al Ghul found the Joker in his cell, Joker coughed up a piece of paper with the formula on it.
After he mutated himself and the other Arkham inmates, the Joker also discovered that mixing Joker Venom with mutagen would not only transform the victim, but also make them violently insane. He used this mixture to mutate Batman, but the effect of the Joker Venom was negated when the mutation was reversed.
Large quantities of Joker Venom were produced at Ace Chemicals by Shredder and Ra's al Ghul, and some of the substance was loaded onto a mutagen-disseminating device to be mixed with mutagen and rained down on the citizens of Gotham City. However, the mixture was ultimately destroyed, as was the supply of Joker Venom.
The Shredder was thrown into a large vat of The some chemicals that created the joker and was disfigured the same way the clown prince was.