Renuyu was a dangerous and addictive chemical developed by the laboratories of Roland Daggett.
Its exact composition is unknown; however, it is known to generate considerable addiction on the subject when it has been used for only a brief time. Part of the initial boom of Renuyu came, however, from the fact it allowed for temporary shapeshifting, presented as a miraculous form of skin conditioning; in reality, Renuyu loosened the skin cells so much it allowed to shape the facial features the user desired, as seen with Matt Hagen. Also, it acted as powerful drug allowing for complete shapeshifting and mass regeneration, though to act thusly it required to saturate the cells of the subject (or in Hagen's case, victim) with a massive overdose of Renuyu. However, these tend to break down after some time and melt down the entire system, unless an untold number of chemical reactions are undertaken.
Roland Daggett saw in Renuyu his ticket for absolute success, though it was flawed and required testing. As the flaws could not be corrected and Renuyu was still addictive and potentially dangerous, he tried to market it anyway. Seeing that he could not hope to do so with his own resources, he tried to introduce the cream into the market by acquiring Wayne Industries by insider trading and using its superior market to introduce the drug to Gotham households, ultimately resulting in the creation of Gotham's resident shapeshifter: Clayface.
In the era of Batman Beyond more than 40 years later. A criminal named Falseface had the ability to perfectly mold his face into any he wanted using his hands. It is entirely possible (but not confirmed) that his ability may have came from using a more perfected form of the same substance.