from the game MASS EFFECT 2
"The Human-Reaper was an incomplete Reaper being constructed by the Collectors. Under the orders of Harbinger, the Collectors began abducting humans from colonies across the edge of the Terminus Systems, taking them to the Collector base.
Once at the base, the abductees were stored in organic pods. Periodically, they would be broken down into raw genetic material which was pumped through tubes to the centre of the Collector base and used to construct the Human-Reaper. EDI suggested that the Collectors were 'facilitating Reaper reproduction' by constructing a new Reaper, and also estimated that the Collectors would have to gather millions of humans in order to complete it. EDI also speculated that any Reaper is built to resemble the race used to create it, and that the Reapers had unsuccessfully attempted to construct a Reaper from Prothean raw material.
When Commander Shepard discovered the Human-Reaper, it was an inactive larva, supported by four tubes. These tubes funneled genetic material into the Reaper and also served as supports. After Shepard's squad destroyed all four tubes, the Human-Reaper fell into a pit to its apparent destruction. However, it was able to activate itself and climb back up to engage Shepard and the team. They proceeded to destroy the Human-Reaper once and for all by damaging specific points on its armour."(wiki)