The Reapers are a highly-advanced machine race of synthetic-organic starships. The Reapers reside in dark space: the vast, mostly starless space between galaxies. They hibernate there, dormant for fifty thousand years at a time, before returning to the galaxy.
These giant machines are ancient; their true name is unknown. "Reapers" was a name bestowed by the Protheans, the previous galactic power fifty thousand years before, and the Geth refer to them as the Old Machines. In the end, the Reapers spare little concern for whatever labels other races choose to call them.
Rendered in 3d Max 10
Background Nebula is a modified NASA image
Post work in Paint Shop Pro 7 & Adobe After Effects CS4
Reaper models by Bioware
Actually it's not exactly a unique trope. I think I've read other stories with a similar theme. But yes it isn't too common... yet. But it won't be long before other, lesser talents adopt the idea and run it into the ground just as they did other concepts like orcs, time travel, and invaders from other planets and everyone is doing it.
Shadows came first.
But i think the motiv for these two races are different. The shadows wanted to promote the development of the young races through conflict. They believe that only the strong are allowed to survive. The Reaper, on the other hand, actually want the exact opposite.They wipe out the most evolved species every 50,000 years to make way for the younger ones.
I would set my money on the Shadows. At least as long as the Reapers fail to indoctrinate the Shadows (or their telephatic pilots).The real strength of the Reapers is that they corrupt the opposing forces and let them fight for them.
I protest! (Sorry, I've wanted to use this in conversation for a long time.) Reapers had spent hundreds of thousands of years harvesting the races of a galaxy, increasing their numbers and assimilating their technology in the process. I have not seen Babylon 5 to my misfortune so I cannot make a fair judgment with the Shadows, but my point is that although one of the main weapons of the reapers is the subterfuge, they still have a wide margin in terms of army and weaponry. for a war on a galactic scale.
The Shadows are also masters of subterfuge, and the only defenses shown to be able to resist their weaponry is Vorlon-derived shielding. Shadow vessels are also ridiculously maneuverable and their method of FTL makes them impossible to detect before they attack and after they retreat. And while Indoctrination takes a toll on the mental faculties of its victims, the Shadows' pawns are made an offer that they are unlikely to refuse.