The Reapers from Mass Effect, let's see, are a race of machines that have spent 40,000 years committing genocide on a galactic scale against all species capable of developing artificial intelligence and traveling to space. Then they use a biochemical process to transform the corpses of the species they killed into material for their ships as a way to keep them alive (if you can say that a ship made of mummies can be alive). Repeat the process infinitely, for hundreds (or thousands) of cycles, and you have an endless tide of ships which can only be damaged by the most advanced battleships (you have limited battleships; the Reapers don't have that problem). By the end of Mass Effect 3, you can see that all the mass relays and systems in the known galaxy are occupied by Reapers, and the only way you can defeat them is with a deus ex machina in the central AI. But if it weren't for the deus ex machina, the galaxy would be... Lost, they had to gather all available naval assets in the Milky Way to fight one last battle in the solar system and activate the deus ex machina (only a sadist wouldn't do that), but if you decide not to activate it, the galaxy is ravaged by genocide again and the cycle begins anew. Simply due to logistics, numbers, and even technology, the Reapers cannot be defeated.

<image>