Metamorphs

When Gong discovered Metamorphosis--true Metamorphosis--it was one of the greatest scientific achievements the Empire had ever known. But it was also one of the most dangerous. The Celestial way of life was defined by the barriers of race and caste. By discovering a procedure that could destroy these barriers entirely, Gong threatened the very foundations of Imperial authority. That was why his execution was so swift and brutal, and, to this day, becoming a Metamorph is one of a tiny clutch of offences that requires disintegration as part of the death sentence. 
Eveline Jin, A Brief History of Celestial Crime

130 years ago, a Permuter from the now disgraced Jin-Gong family invented an Arcane Genetic Modification that, although often fatal, allows the recipient to change their form at will to any appearance they please. The risk that this ability presented to the natural order led to the procedure being banned, and Gong Eighteenth was publicly executed.
Despite its inventor's death, the details of the procedure were spread amongst elite Permuter circles, and those brave enough to undertake the procedure--usually those who face a dire fate otherwise--have become faceless, losing all racial characteristics as they transform. These people must live without identity, becoming effectively Casteless; but if they are ever discovered, they will inevitably face death for their crimes.