Alright, so basically you are after a creature called Leviathan that has killed at least one Reaper. It appears to use indoctrination as well, but over long distances through these almost organic, water artifacts. Turns out, the Reapers are hunting it as well since they perceive it as a threat. You search through different planets and eventually discover that it is located on a planet mostly filled with water. You fight off Reaper forces and use a Dive Mech you discover to plunge into the depths of the ocean in order to find it. Eventually you do and find out that there is more than one, and that they existed before the Reapers.....in fact, they created the Catalyst which was to solve the problem of synthetics rebelling against their creators. The reason they did this was that they were the "Apex Race" and had the galaxy in their thrall (i.e. slavery/servitude) and wanted to protect them from their own creations. The Catalyst turned on them and harvested them to create the first Reaper (i.e. Harbinger) which was made in their image. The ones you discover escaped the first harvest and managed to remain in hiding, gaining glimpses into the universe through the artifacts they use to control humanoids. In the end, you convince them to join the fight and help you, so another War Asset to the mix!