Shadow Master is a non-playable character in Soulcalibur III that is only fought in a Prepare To Defend Yourself challenge in Proving Grounds in Tales of Souls, though he only appears if the player follows a certain path.
Possible relationship with Kilik
Kilik has a master named Edge Master. As both Kilik and Shadow Master are fought in the Proving Grounds, Shadow Master could be an evil side of Edge Master or a split personality, or a rival, this is never mentioned in the Soul series. Note that can also be Edge Master as "unknown soul".