Getty Images We now have a confirmed cause of death for Buffy the Vampire Slayer star Nicholas Brendon. We learned that Nicholas passed away on March 20. He was only 54 years old. Now, Putnam County Coroner Todd Zeiner has completed his autopsy and has revealed the official cause of death. Photo Credit: Getty Images He apparently passed away from "atherosclerotic and hypertensive cardiovascular disease with acute pneumonia and previous myocardial infarction being contributing factors," Page Six reports.