Smoke Demons were dark side entities that could be summoned through Sith sorcery. They could metamorphose into their targets' worst fears and sought to vaporize their bodies. At some point following the Battle of Yavin, the S'kytri Dark Jedi named Kharys used a Smoke Demon to murder the smuggler Katya M'Buele.
A Smoke Demon, called Siqsa in Sith, was an insubstantial manifestation of the dark side of the Force that assumed the form of its victim's worst fear upon being summoned by a dark-sider. They could be summoned through an ancient Sith ritual recorded as Dwomutsiqsa—literally "Summon Demon"—in an ancient scroll recovered by Sorzus Syn.
The creatures could absorb their victim's body by flowing into it and vaporizing it, entrapping within them the victim's soul, which would be released upon the Demon's destruction. They would first materialize as mere spirals of smoke before assuming a definite shape, then would paralyze their targets, leaving them conscious but helpless. Then the demon flowed into the body of its victim, who turned translucent, then transparent, and finally faded away. The entire body was turned to smoke and absorbed by the demon.
Despite being made of a smoky substance, a Smoke Demon apparently had significant strength, and was able to defeat a Wookiee in hand to hand combat. It evidently did not have a capacity to speak orally, but instead communicated through some form of psychic ability, the extent of which is unknown.
Following the Battle of Yavin, one such demon was used as an assassin by Kharys, the violent ruler of the planet Skye, due to its ability to move and kill silently. The S'kytri had learned the Dwomutsiqsa ritual under the tutelage of the Dark Lord of the Sith Darth Vader, following the end of the Clone Wars. The demon summoned by Kharys eliminated the female smuggler Katya M'Buele while she was sleeping aboard the Millennium Falcon, with others standing guard. However, the demon was not able to escape, as he was impaled by Luke Skywalker's lightsaber, which harmed the creature whereas blaster bolts did not. Skywalker was deeply affected by this encounter, and the young Jedi immediately recognized the mark of a powerful dark side user.
- Book of Sith: Secrets from the Dark Side