Harbinger

Harbinger was a Reaper that resided in dark space with the rest of the fleet and the first of its kind ever created. It was directly responsible for the Collectors' operations, frequently possessing individual Collectors to fight battles in person, and it took a particular interest in Commander Shepard. That interest first surfaced shortly after the Battle of the Citadel, when Harbinger struck a deal with the Shadow Broker to obtain Shepard's body after the destruction of the original Normandy. The attempt failed, and Shepard was instead resurrected while Harbinger advanced the Reapers' plans. (Mass Effect 2; Mass Effect 2: Lair of the Shadow Broker; Mass Effect 3: Leviathan; Mass Effect: Redemption)

Master of the Collectors#

Harbinger was the mastermind behind the Collectors, controlling them with the Collector General as its proxy and directing them to abduct entire human colony populations so that the captives could be processed into a Human-Reaper. While controlling the General, Harbinger could assume direct control of any individual Collector, often doing so when the Collectors fought Shepard. Through such a Collector it could speak, commanding other troops or threatening its enemies, and the possession made the chosen Collector far more powerful. The process appeared painful to the possessed drone, which always disintegrated when defeated, an effect much like Sovereign's possession of Saren's corpse. (Mass Effect 2)

It showed a personal interest in Shepard, ordering the Collectors to recover the Commander's body intact when possible. (Mass Effect 2)

The end of the Collector campaign#

After Shepard's team assaulted the Collector Base and destroyed the still-incomplete Human-Reaper, Shepard could either destroy the base outright or purge it of all life. In either case Harbinger proclaimed Shepard's failure to stop the Reapers, declaring that humanity already had their attention. It then began relinquishing control of the Collector General and vowed that the Reapers would find another way, after which it was seen with the rest of the fleet in dark space, moving toward the Milky Way. When Shepard later acted to delay the Reapers' arrival, Harbinger appeared by hologram to speak of the futility of the fight, or, if the Collectors had not yet been defeated, spoke through the General instead. (Mass Effect 2; Mass Effect 2: Arrival)

The assault on Earth#

During the Reaper War of 2186, Harbinger was one of the Reapers leading the attack on Earth, and Alliance intelligence judged it the oldest and largest Reaper of the entire fleet. It appeared during the final battle in London, launching a devastating attack with weapons that fired faster than other Reapers' against the allied forces trying to reach the Citadel through the conduit beam. It withdrew when it seemed Shepard and the counterattack had been wiped out, yet Shepard and David Anderson made it through, both gravely wounded. (Mass Effect 3)

If Shepard activated the Crucible, Harbinger shared the fate of every Reaper: destroyed with all synthetic life, brought under a unified control to rebuild and guard the galaxy, or, after the merging of organic and synthetic life, willingly helping to rebuild and share its knowledge. Should Shepard refuse, the Reapers won the cycle, and they were left to be defeated by a later cycle that found the warnings Liara had left behind. (Mass Effect 3; Mass Effect 3: Extended Cut)

Origin among the Leviathans#

When Commander Shepard encountered the surviving Leviathans, they revealed that Harbinger had been the first Reaper created and that it had been formed in their own image. The Catalyst directed the Reapers to harvest the galaxy of advanced life roughly every fifty thousand years, and each harvest culminated in the birth of a new Reaper modeled on Harbinger's form. In this way the eldest Reaper was both the origin and the enduring template of the cycle of extinction. (Mass Effect 3: Leviathan)