Human-Reaper Larvae

The Human-Reaper Larvae was a monstrous creation of the Reapers that was a fusion of human DNA and Reaper technology. It was made by the Collectors via mass-harvesting of humans who's DNA was then fatally extracted and turned into the material used to create a techno-organic abomination in the vein of the Reapers themselves. Unfortunately for the Collectors, the Larvae had not yet been quite finished by the time Commander Shepard and their allies stormed the Collector Base. Confronting the Larvae at the very end of the mission, Shepard and company wasted no time attacking what they recognized as a threat to the galaxy, and managed to destroy it despite the Collector's efforts to protect it and also the Larvae's own attempts at combat.

Desiring to control the Reapers, the Illusive Man later had the remnants of the Larvae harvested and shipped to his base on Chronos Station. Depending on whether or not the Collector Base was spared, either the Larvae's "heart" or "brain" was successfully recovered. Commander Shepard would later take the recovered piece to use in the war against the Reapers.

Trivia

 * The Human-Reaper Larvae bears some resemblance to a Terminator Endoskeleton
 * The Boss fight with the Larvae is widely considered far too easy and completely anti-climactic