A Void in the Force, or Force Blank, was a rare and nearly unheard of phenomenon, in which a living entity seemed to be fully nonexistent in The Force. Such a being could neutralize his appearance and aura so completely that those who met him could not recall what he looked like.
The earliest known examples of this event are the castings of the Sith Emperor's ritual that he used to consume life in order to maintain his immortality, both from his planet of Nathema, and later attempted from the entire galaxy. When the Emperor feeds, he creates a complete "void" in the Force, and takes away color and nearly all sound in the area.
In 27 BBY, Granta Omega, the son of the Dark Jedi Xanatos, was described as a "void" in the Force by Obi-Wan Kenobi, who himself could only speculate Omega was utilizing some form of self-concealment technique. Peculiarly, Omega's ability allowed him to escape detection by the Jedi, and also rendered him extraordinarily difficult to remember. Even in the event someone did recall him vaguely, they would not be able to describe his appearance in any manner that was exact or precise.
Another incident of this marvel was identified in 0 ABY, concerning the Human clone assassin X-7. As noted by Jedi Knight Ferus Olin, X-7 was noted as having a hollowed body, with the Force flowing around him, rather than through him. This was the first and only known incident of such a phenomenon occurring artificially.
Perhaps the most outstanding cases were those of the assassin Umbaran siblings, Nyss and Syll Nenn. Both siblings displayed the ability to create a dark void in the force, making them undetectable to Force sense. Further still, the Nenn twins could also make the Force inaccessible to Force-sensitives (though Jaden Korr and his clones were powerful enough to overcome this for unknown reasons). Indeed, this power was potent to the degree that they were capable of nullifying the force energy present in some lightsaber crystals, causing them to shut off completely. After the death of Syll, Nyss Nenn felt that his ability had increased, when while his sister was alive, he had always psychologically thought that she was an amplifier.
The phenomenon of a Force void must not be confused with the Force power known as Force cloak. Whereas the former was an inherent ability found only among beings with natural leanings toward the dark side of the Force, who gravitated or were naturally drawn to it, the latter was a talent or an acquired skill predominantly among practitioners of the light side. Both phenomena were very rare.