The Reapers are a synthetic-organic machine race of starships whom reside in the void between the galaxies. Where they hibernate for 50,000 years, return to the galaxy to harvest it's advanced civilizations and species, return to the void to hibernate for another 50,000 years, and repeat the cycle. Ancient machines which are thousands or millions of years old, with the oldest, the Harbinger having been around for 100s of millions of years. Originally created by the Leviathans to preserve all life, whom they turned on and nearly exterminated.
Reaper ships resemble somewhat the squids or cuttlefish of Earth, with tentacle-like legs on a semi-cylindrical form that has a tapering plate at the rear. The ships range in sie from 160 m in length to over 2 kilometers. There are various sub-types with variations on the general shape, though the core of every ship is constructed in the images of the harvested species.
The Sovereign-class Reapers which are the largest sub-type are the capital ships of the Reapers. Equipped with a "magnetohydrodynamic" cannon. A weapon that dwarfs the main guns of the most powerful ships of the galaxy's interstellar powers, delivering 132 to 450 kilotons of TNT per round. Weapons that easily tear through the hulls of the mightiest dreadnoughts. The ships also have shields that deflect the attacks of entires fleets.
The Reapers are able to manipulate the minds of any organic being within close proximity and through specific artifacts. These indoctrinated sentients willingly serve the Reapers, eventually becoming mindless slaves. Through this indoctrination, they've taken over entire civilizations, through the actions of a few important individuals. For ground combat, they deploy the "Husks" whom are mindless aggressive drones created from organic beings. Whose bodies have been taken over by cybernetics. They are the Reapers' shock troops, overwhelming enemies in waves.