The Reaper called Harbinger is believed to be the oldest and largest in the Reaper armada. From the reaches of dark space, Harbinger managed to control the Collectors, a race of human-sized insectoid bipeds, as it sent them on a campaign to kill and gather humans from vulnerable colonies. The Collectors became a terrifying force in the galaxy, responsible for the murder of hundreds of thousands. Surviving colonists have described the tone of Harbinger's threats, heard through the Collectors as they attacked, as visceral and terrifying.
Alliance intelligence has tentatively identified Harbinger as one of the Reapers leading the attack on Earth.