Complex sentence: As the Pianist sees Sybil talking to Norbert, he attacks Louis in a fit of rage, who is still completely unaware of his former relation with Sybil.
Split into simpler sentences: As the Pianist sees Sybil talking to Norbert, he attacks Louis in a fit of rage. Louis is caught off guard as he is still completely unaware of the Pianist's former relation with Sybil.