Force Phantom

Known to be an elaborate Sith technique, Force phantoms were lifelike apparitions that when created, could fully interact with the physical world around them in whatever manner the conjurer desired. This arcane ability was employed by the ancient Sith Lord Darth Vectivus, and discovered (not recreated) by the Blackguard known as Velok.

Locating a host
The creation of Force phantoms was physically taxing, and required immense mental concentration. By withdrawing deep into the Dark side of the Force, the conjurer had to locate the aura of an individual through the Force, usually someone who the conjurer was in some manner familiar with. Although not necessary, it helped if the conjurer was aware of the target individual's exact location in the galaxy. To the inner eye, the target's Force aura manifested itself as a distinct yellow glow, surrounded by red sparks of energy. If the target was aware of the conjurer's influence, their efforts to reduce the conjurer's influence would evidenced by a lesser manifestation of the telltale red sparks. Once found, the conjurer rooted their own consciousness in the target's mind, and prepared for the next stage of this complex ritual.

Creating the phantom
After successfully anchoring themselves within the mind of their target, the conjurer would then seek out the mind of a new individual. The auras of other individuals did not produce the same red sparks as the original target. At great distances, stronger and more alert minds were more difficult to influence, and those of the weak were less coherent and therefore difficult to touch as well. By sampling the minds of these various beings, the conjurer could also decipher both the species and gender of their target. Sleeping individuals were ideal targets for this part of the ritual. When a suitable target was located, the conjurer would then affix themselves the new individual's mind in a parasitic manner, and use both the Force and physical life energy of the unsuspecting target being to draw the conjurer's own Force aura into the target's actual location. Now themselves appearing as an apparition, the conjurer could fully perceive and manipulate their new surroundings through their own Force aura, while still being mentally anchored to both the original red spark-identified target, as well as the secondary individual.

The secondary individual whose mind was being intruded upon could be controlled by the conjurer in the manner of a zombie, being themselves unaware that they are being controlled by outside means as they do the conjurer's bidding. When the conjurer's will had been satisfied, the host would be returned to the state that they were found in, usually completely unaware of what had transpired. Should the conjurer choose, they could transform themselves into a form that was recognizable to those whom they intended on interacting with, so as to deceive their target with a false sense of familiarity. Any injuries that befell the phantom were reciprocated to the target, and should the phantom be destroyed by an outside force, the target individual would die.

History
The ability to create Force phantoms rested with an ancient Sith Lord known as Darth Vectivus, who resided in an immense asteroid called The Home. Due to prolonged exposure, his asteroid became over time permeated with the dark side of the Force. It was abandoned after Vectivus' death and never rediscovered. Velok discovered the technique in journals belonging to one of Vectivus' students, and after considerable experimentation was able to use the technique successfully when near a Force nexus.

Information from Wookieepedia.