In the Mass Effect series, the Reapers are the ultimate enemy - a predatory race of ancient machines that waits in dark space to harvest spacefaring species every 50,000 years.
The Reapers have been around for a very, very long time, and have shaped Mass Effect's timeline in many ways. Originally created by the Leviathan race well over a billion years ago, their creators were the first race they harvested. They’ve been around at least that long, because the Leviathan of Dis (which is actually a Reaper corpse, not the same as the Leviathan race) is around one billion years old.
Assuming the Reapers have been around for one billion years, and assuming they’ve harvested a species every 50,000 years without a break, the Reapers have harvested aliens from 20,000 cycles. Assuming about five alien races every cycle (and even Shepard’s cycle has more than that), that means they’ve harvested at least 100,000 alien species. Yet, in-game, the only race Shepard hears about before the current cycle is Mass Effect's Protheans, the alien race harvested before humans came along.
But in the details about the planets Shepard can visit while they're exploring Mass Effect's galaxy, there are traces of civilizations that came before the humans and the Protheans, civilizations that were later wiped out by the Reapers. Note that there are several planets in Mass Effect that show signs of having once been inhabited. This list is of the planets that show signs of Reaper activity - namely, mass extinctions, attacks from space, and mysterious disappearances of entire species.
Mass Effect: Reapers On Helyme
Helyme is a planet in the Crescent Nebula that Shepard visits for a side quest in Mass Effect 2. It’s described as a “post-garden” world that was once the homeworld of a race called the Arthenn, who vanished 300,000 years before Shepard was born. The description of the planet reveals no one is exactly sure what happened to the Arthenn, but Helyme suffered a cataclysm that wiped out all vertebrate lifeforms. If that wasn’t enough of a clue, the nearby planet Epho, which also shows signs of having been inhabited by the Arthenn, has massive craters left over from orbital bombardment.
Mass Effect: Reapers On Joab
Joab is a planet in the Rosetta Nebula, also the site of a side quest in Mass Effect 2. As the description reads, the planet is best known for its mysterious mass extinction event. There is no name for the species that once inhabited Joab, only that it was “primate-like.” Everything scientists know about them comes from time capsules, as all habitation centers on the planet show signs of having been obliterated by orbital bombardment. This kicked so much dust into the atmosphere that all life, including plant life, was wiped out.
Mass Effect: Reapers On Aphras
Aphras is an outwardly unremarkable Mass Effect planet in the Shrike Abyssal, which Shepard might miss - it just happens to be in the same system as yet another side quest. According to the description, there is fossil evidence to suggest the planet was at one point inhabited by a sapient, avian species.
However, this species was decimated out by a series of massive impacts from space that wiped out all life on land. Scientists initially thought this was a random event, but later discovered that the impacts were all on inhabited areas.
Mass Effect: Reapers On Bothros
Bothros is an easy-to-miss planet in the Hades Nexus, which borders geth space in Mass Effect. It doesn’t appear habitable, being primarily made of ice and rock. Scientists discovered, frozen in the planet’s ice, a spacefaring species capable of wearing protective suits. The of the species frozen were the lucky ones, as they were only frozen because they were outside the planet’s habitation centers. The planet’s cities were destroyed by orbital railgun weapons that hit the planet’s surface with the force of 120 kilotons of TNT. There’s no way to know for certain if the species originated on Bothros.
Mass Effect: Reapers On Etamis
Etamis is a post-garden world, the oceans of which have been vaporized by orbital bombardment by kinetic weapons. The only traces of civilization that scientists have found suggest an advanced, spacefaring species lived on the world between 20 and 40 million years before Shepard was born. Interestingly, this time frame means it’s possible this species is the one that fired the mass accelerator weapon that the Illusive Man speaks of, which scarred the planet Klendagon 37 million years prior and killed the Derelict Reaper Shepard visits in Mass Effect 2.
The Reapers have left scars all over the galaxy, and the player can find them if they keep a sharp eye out. The details in Mass Effect are one of the many things it has to offer players.