Terrak Morrhage's Master

The "Unindentified Sith Master" was the former Sith Lord master of Terrak Morrhage.

History
During his time at the Academy of Sigil, Terrak Morrhage was under the tutelage of an unknown female Sith Lord, who was determined to rise him to greatness. The master and apprentice worked together to create hazards for their greatest enemy: the Jedi Order.

When the master suspected that the order was looking for her, she fled Sigil in fear, but she was eventually caught and executed.