system
Given the following post, respond as if you're commenting on Reddit:
human
[Mass Effect] Can someone explain The Reapers to me in simple terms? Like who 'built' them, if that question makes sense? What are they harvesting? Are they getting stronger, weaker or staying the same over time? Are there more of them/less/same? Just "what" are they?
gpt
A long, long, long time ago there was an alien race that had made huge advances in AI/synthetic life/robots. They were worried about the possibility of a robot revolution so they put one of their AI (named "Catalyst") to work calculating a good way to prevent it. 

Catalyst's solution was... not ideal. It created the Reapers, a fleet of sentient starships, each one made from a substance that was literally processed from thousands of slagged down organic life forms. 

Then, because it was what they were programmed for and all they knew how to do, the Reapers decided to do that across the galaxy. When they were done they hibernated for eons and waited for the re-emergence of intelligent life with its own AI buddies; then they did the whole thing over again. And again. And again. And again. 

They got really clever about it, building the Mass Relays and the Citadel to lure sentient life into one concentrated place to make their work easy. By the time Shepard comes around a countless number of cycles have already been carried out.