The Harbinger was the first Reaper created by the Intelligence. The Harbinger oversaw the Collectors, and sometimes even possessed individual Collectors to suit its needs.
The Harbinger, like all Reapers, primarily lived far outside the Milky Way in dark space.
History[]
The Harbinger was the first Reaper, created by the Intelligence out of genetic material taken from Leviathans.
In 2985 CE, the Harbinger had possessed the Collector General and, through him, had the Collectors abduct entire colonies of humans with the aim of creating human-Reaper hybrids.
When Shepard began combating the Collectors, the Harbinger would occasionally take direct control of individual Collectors to speak to their opponent. When released from possession, the Collectors always disintegrated. The Harbinger bore a special interest in Shepard, and sought to claim his body -- a consequence of Shepard's defeat of Sovereign.
Ultimately, Shepard managed to activate the Project in order to delay the arrival of the Reapers. The Harbinger appeared before Shepard in a holographic projection at this point, directly warning him that the Reapers were still coming and fighting against them was ultimately futile.
In 2986 CE, the Harbinger led the assault on Earth, and was only stopped when the Crucible was activated and the Reapers fell under the control of Commander Shepard.
Source[]
The Harbinger is derived from Mass Effect.