Unknown Soul is a foe that is fought in the Tales of Souls mode in Soulcalibur III. It can be fought in the Grand Labyrinth, in which Olcadan resides. It assumes one of three different fighting styles: Hwang, Li Long, and Arthur.
Unknown Soul is textured all black - with no extra abilities. The arena in which it fights is also darkened, which makes tracking it in battle more of a challenge.
It is likely that Unknown Soul is merely the character being represented, but shrouded, rather than a separate character. The fact that the stage is darkened supports this, giving the impression that it is simply supposed to be too dark to see the character clearly.