Centuries ago, Mastrius was a powerful vampire that terrorized the surface world for years until the Daedra Prince Azura intervened. Azura cast a spell on him, trapping him inside the dungeon and draining him of his strength. Mastrius has been trapped ever since, waiting for someone from outside to come find him and set him free.
Mastrius wears a near complete set of Ebony Armor with the exception of the Ebony Closed Helm. He wields a Demon Katana when in combat. Mastrius is of the Quarra bloodline, although he is not considered to be a part of the clan itself.
He was held against his will by Azura, and asks the Nerevarine to set him free in exchange for shared world domination. He will ask for two things; a Daedra's Heart and the artifact Spell Breaker. Once the items has been returned to him, he will ask the Nerevarine to borrow some strength so he can break the binding spell. Once he is fully released, he will for attempt to attack the Nerevarine while in a weakened state.