Mask (Sith Alchemy)

The Mask technique was a special application of Sith Alchemy, a technique that was little-known in the galaxy (perhaps even almost forgotten)- for good reason. Though Sith Alchemy could be used to create dangerous artifacts or horrific monsters, the Mask technique was far more subtle. A Sith Alchemist could use the Mask to literally reshape an individual's appearance, altering a body at the molecular level. Very little was beyond this technique, limited only by the craftsmanship of the user. Features, age, disfigurations, even species could be concealed with the Mask (though the latter would require an incredible amount of skill to accomplish convincingly). Owing to its molecular nature, the technique lasted until altered, and even a thorough medical examination would not be likely to reveal anything out of place, unlike the tell-tale scars of surgical alterations. In fact, not only was a Mask more convincing than cosmetic surgery, but it was also horrifyingly faster: a skilled alchemist able to alter an appearance in mere minutes (provided the necessary materials were on hand).

Normally, this technique was used by alchemists to conceal the ravages of the dark side, or to hide their identities from inquisitive Jedi, but it could be used for far more sinister means... such as duplicating and then replacing a chosen individual.

The technique wasn't entirely without risk. The process itself could be incredibly painful, and complications from the procedure could result in serious injury or even death. There was also one rarely-encountered oddity with the Mask technique. As it was a product of the dark side of the Force, on rare occasions it would resonate when exposed to powerful blasts of dark side energy. If the subject was not careful, the Mask would literally melt away, horrifically disfiguring the individual and making further attempts at concealment (even with the Mask) difficult.

It is said that Palpatine, a master at Sith Alchemy, may have used this technique in order to conceal his appearance from the people of the Republic and present a more appealing facade. But if he did, his encounter with Mace Windu and several other Jedi Masters changed that, when Mace reflected Palpatine's lightning blast back at him, disfiguring the Sith Lord's face.

Behind the scenes
The "Mask" ability was proposed by the Star Wars Roleplaying Game as a method of resolving the debate behind Palpatine's transfiguration during his battle with the Jedi Order. Sources had suggested that Palpatine had been hiding his true features until the battle, and when combined with the damage from the lightning, the result was the disfiguration seen on screen.