The reapers are an ancient AI that are tasked by an unknown maker to save the galaxy from itself. As other civilizations become more and more intelligent, they infringe on those other species making them unable to develop into an advanced civilization. So the reapers destroy the species that has grown too much so less developed planets have a chance. In this game humanity has reached a new level of development and now is part of the galactic council, but of course this ushers the reapers to wipe every advanced, sentient race in the galaxy. This includes humanity.
At first, a reaper could be mistaken for a huge, squid looking mother ship but the ship is a reaper. The first reaper we see is Sovereign threatening to destroy the council by himself. Since this is the first reaper we see, it comes as a shock to see such a thing. I enjoyed the lore behind this game and I thought the reapers were especially cool. They felt unstoppable and a force that can never be beaten, but that is all. Thanks for reading, let me know if you enjoyed, and I'll see you in the next one.