Hwaitees:

The Hwaitees are a race which has evolved to adapt to their surroundings by constantly shedding their flesh and skin, from prey to predators they have acquired an evolutionary trait that makes them quite unique. The ability to metamorphose to any kind of species they have devoured. A simple pound of meat, some bones or the blood of their victims is all it takes for them to acquire the ability. The transformation is neither fast nor easy. It takes time and a lot of energy, the time and energy heavily depends on the appearance they must adopt and once taken they do not take upon their skills (Strength, magic, etc), much akin to a costume made of flesh and bone. Their ability to change appearance is a closely kept secret as its their main tool of infiltration into foreign societies, as such Hwaitees take the form of sirens, ancient natives of the island they now inhabit and which have been transformed into mindless livestock after generations of being devoured to their children so they could take up their shapes.