There are only 500 hundred of them spread out all over the world. All of which were chosen after death to become Reapers. Reapers, that's what they are. They are the ones who take your lives away from you once your hourglass runs out. They are your cause of death if no one gets to you beforehand. They live in human world along with you but they are to be no relationships with you, Humans. Their job is just to make sure you die the right way. The way that was written on your hourglass since the day of your existence. No other magical being should have an effect on how long your life is. If they do, it is up to the Reapers to destroy them. Their lives also rest in our hands. It's their job, like I said they were chosen after death. Lord Death chose only the messed up ones to give the option of becoming a reaper; the ones who had their lives corrupted by every dark source out there.