My theory about the Phantom Slasher
I think that the Phantom Slasher is the “Perfect Blade” that Kalmar was talking about in the Inner Sanctum. Traziun thought the Phantom Slasher’s mission was impossible so he ran away and Gaius became the Phantom Slasher instead of him. Kalmar said that Traziun were to be the instrument of perfect justice which means that the Phantom Slasher’s mission is to rid the way of all evil.
I also think that both Rhue and Gaius has an Illuminati, Gaius even seems to be able to sense where the other Illuminatis are and what they does and that’s probably why he never attacks Rhue in the beginning, helps/protects Rhue and knew that Rhue killed Lyrra.
I was going to put a list on why this theory should be right but my English turned out to be worse than I expected so I'll just leave it as this.