Nicholas Brendon's cause of death has been revealed. As we previously reported, the Buffy the Vampire Slayer alum passed away at 54 in March. His family broke the sad news, saying he died "in his sleep of natural causes." The Putnam County Coroner has now confirmed what happened to Nicholas. According to a report obtained by TMZ on Tuesday, the actor's death has been ruled natural. He died from atherosclerotic and hypertensive cardiovascular disease, which the American Heart Association says is caused by plaque buildup in arterial walls.