Face Dancers are a type of human in Frank Herbert's Dune universe. Servants of the Bene Tleilax, Face Dancers are near-perfect mimics; their name is derived from their ability to change their appearance literally at will.
Originally, Face Dancers were Tleilaxu trained to mimic others using acting and makeup, enhanced by plastic surgery. As time went on, the Tleilaxu began to use genetic manipulation to enhance natural ability, so that Face Dancers could change height, increase and decrease apparent mass, change colouring and texture, and change facial features. This genetic manipulation continued to such an extent that Face Dancers could be considered a sub-species of humanity or even another species entirely.
In the history of the Dune universe, Face Dancers have been employed for a number of purposes. Initially, their principal use was entertainment--they served as comedians, impersonators, and actors. However, their identity-stealing skills made them highly useful as spies and assassins, and they were hired by the Great Houses in their continual political, financial, and military conflicts.
In time the Face Dancers became genetic eunuchs. They became sterile creatures, with full sentience but no sense of self and a genetically programmed loyalty to the Tleilaxu masters. They can be controlled by being forced into a hypnotic state with some predefined sound (often a specific whistling noise).
Throughout most of the Dune timeline, only the Bene Gesserit-trained can detect a Face Dancer by sight; Face Dancers are liable to give away their identities, though, since they lack the memories of the people they replace.
In Dune Messiah, the second book in the series, the plot is driven by a Tleilaxu attempt to undermine the Emperor Paul Muad'dib using a Face Dancer, in the hope the Tleilaxu can take control of the Imperium.
At the beginning of Heretics of Dune, the Bene Tleilax feel ready to take control of the Imperium. They have achieved their long-term plan of developing Face Dancers who are perfect mimics, able to take mind prints of the people they imitate and possess all their memories. This leaves an ordinary person with no real way of detecting a replacement; the Tleilaxu believe that the Bene Gesserit cannot detect these new Face Dancers either. The Tleilaxu intend to take control of the other powers in the Imperium by replacing their leaders with Face Dancers. However, the Tleilaxu plan is ultimately ruined by its flaws. The Tleilaxu have never tested the Face Dancers over long periods independent of a master's control. It develops that, after playing their roles for too long, the new Face Dancers come to think of themselves as the people they have printed and forget their Tleilaxu origins. They effectively become the people they are mimicking, passing beyond the control of the Tleilaxu. Nor are the new Face Dancers undetectable to the Bene Gesserit. By the end of the novel, it looks as if the Bene Tleilax will be forced into an alliance with the Bene Gesserit.In Chapterhouse Dune, The Reverend Mother Dortujla describes being approached by a group of Futars and Handlers wishing to ally with the Bene Gesserit against the Honored Matres. Dortujla is struck with the impression that the Handlers are Face Dancers, but standard Bene Gesserit detection techniques are unable to confirm her suspicions. The novel also briefly introduces the mysterious Face Dancer couple Daniel and Marty, who are independent of the Tleilaxu and are following an unknown agenda of their own.