Soul Swords, or Reiken as they are sometimes called, are a type of energy weapon in the world of Spectral Shadows used mainly by Omman Knights, though other entities have been seen wielding them, such as the maleficent entity that possessed Christine and brandished a black-colored Soul Sword; Christine herself would later gain the ability to use Soul Swords later on in Serial 11. Soul Swords are powered by one's own spiritual energy, and they seem to be effective against Vampires, as Lucious Rhoades seemed just as fearful of Rael Ommandeer's Soul Sword as he did the sun.
Soul Swords were first mentioned near the end of Serial 1 "Children of the Ommadawn" where Rael Ommandeer first sees them used within a game, and gets Captain Shane, then his Sensei, to teach him the ability. Jon Ommandeer would later learn the Soul Sword ability, as we see him using the power here and there at times in Serial 11. They seem to be powerful weapons, able to cut through various objects and can even cut through living things.
Soul Swords sort of have a double inspiration, as they take their name and form after a power from the anime YuYu Hakisho and also function similar to Lightsabers from Star Wars (to say nothing of lightsabers being the trademark weapons of Jedi, much like Soul Swords are the trademark weapons of the Omman Knights).