Harbinger is the leader of the Reaper fleet, being the largest and oldest of all Reaper capital ships. It first made its presence known in Mass Effect 2 as the controlling force behind the Collectors and led the invasion of Earth during Mass Effect 3.
Eons before the events of Mass Effect took place, the apex race that would be known as the Leviathans created a great artificial intelligence designed to protect all sentient life in the galaxy. This Intelligence (which would become known as The Catalyst in the future) determined that organic life could only be safeguarded by preserving it through synthetic means. Organics that were allowed to develop on their own would inevitably advance to the point where they could create synthetic life and the Intelligence was utterly convinced that all machine life was destined to eventually become sentient and rebel against organics, threatening all life across the galaxy. Thusly, the Intelligence created numerous bio-mechanical pawns to carry out the task of harvesting all organic races that developed sufficiently advanced technology. However, the Leviathans themselves were also deemed as being part of the problem, so the Intelligence's solution applied to them as well. The creations of the Intelligence hunted and harvested the Leviathans to near-extinction and would preserve the combined intelligence and genetic material of the Leviathan race within a new intelligent construct. This new machine would be crafted in the image of the race it was forged from, possessing all of their combined knowledge and directing it towards the purpose of the Intelligence. This was the creation of the first synthetic entity that would one day be known as Reapers. This was the birth of Harbinger.
Mass Effect 2Edit
Throughout ME2, Harbinger makes no physical appearance. It makes itself heard through the Collectors as they attacked Human colonies and the crew of the Normandy, taking possession of the Collector General and channeling its consciousness to one of the many drones across the Milky Way. Harbinger has targetted Humanity specifically for harvesting, recognizing their genetic diversity and their potential for creating a new Reaper. They are also regarded as the most potentially threatening race in the galaxy and are targetted early in order to remove the most chaotic element of the coming Reaper invasion. As Shepard and the Normandy crew battle the Collectors, Harbinger makes several threats to deter the commander from resisting, but to no avail. When the Collector base is destroyed, Harbinger releases the Collector General from its manipulation and leaves it to its fate. It then mobilizes the Reaper fleet in dark space and leads them as they descend upon the Milky Way.
Mass Effect 3Edit
Harbinger has a significantly reduced role in the third game. Although it supposedly leads the Reaper forces invading Earth, it is not seen as they descend upon the planet. In fact, it makes its only short appearance during the final battle on Earth as Shepard and several Alliance soldiers make a push for the Conduit to reach the Citadel. Harbinger unloads a volley of particle beams and vaporizes most of the soldiers, then takes off after Shepard has been knocked out. It has no dialogue whatsoever during the game and does not possess any Reaper ground units as it did in ME2. Whatever the outcome of the game, depending on the final decisions made by the player, Harbinger meets the same fate as the rest of the Reapers.