From Lost Fables Wiki
Bottled rot is a disgusting substance, distilled flesh and fluids from several putrescent corpses, captured in a bottle. Though it naturally will be filled with foul disease, the jars are actually known to generate new diseases, a property sought after by some of the more disturbed types.
To gather bottled rot, all you need to do is get three corpses, a giant cauldron, and a lot of milk. Let the corpses rot for half an age, then toss them in the cauldron and cover with milk. Boil the mixture, continuously stirring and pounding until it is a single consistency. The resulting filth which rises to the top is the distilled essence of rot, ready to be balanced. You can get one jar per 100kg of corpse, generally speaking.
Besides creating and spreading disease, bottled rot has essentially no mundane uses. For alchemical purposes, it can make the effects of potions contagious, and if its effects are reversed, it can be made into a cure-all.