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Isaiah is one of the two Strigoi who kept Rosemarie Hathaway, Christian Ozera, Mason Ashford, Mia Rinaldi and Eddie Castile prisoner in Frostbite. He uses Eddie as a Feeder. Also, he does not provide the five prisoners with food, water, or blood. He has given the two Moroi (Christian and Mia) a choice. One of them may live and the other may die. All the one who wants to live has to do is drink the blood of any of the three dhampirs, Rose, Mason, or Eddie, and become a Strigoi. Isaiah taunts Rose viciously when they are held captive.
Isaiah is incredibly old and incredibly powerful, being the oldest and strongest strigoi seen so far in the series. Isaiah was once a moroi and stands almost as tall if not as tall as Dimitri. His power is seen when he pushes Elena for annoying him and, although he put little effort into it, it was strong enough to throw her across the room with sufficient force to cause injury had her strigoi reflexes not stopped her from crashing head first into the wall. His power is demonstrated again when he takes a number of bullets straight to the chest as if they were simply an annoyance to him. Because of his power, he is incredibly vain, giving off the air of an aristocratic gentleman
Isaiah is responsible for the death of Mason Ashford. Rosemarie Hathaway is responsible for the death of Isaiah with assistance from Mia Rinaldi.