A harbinger is defined as a person or thing that announces or signals the approach of another, or a forerunner of something.

Just like psychopomps, it's generally believed that harbingers don't actually cause people to die, although if they do appear, death is pretty much a given.

The belief that harbingers are terrifying or something to be feared has resulted more from their depiction in pop culture than from their original meaning in folklore and myths.