Orbiting Mnemosyne is a two-kilometer-long ship with the unmistakable profile of a Reaper. It is giving off power signatures in localized areas, but they are far weaker than a ship this size would indicate. The Reaper seems to maintain a mass effect field that has kept it from falling into the failed star, but massive holes have been blasted and melted into parts of the hull and remain unrepaired. The only logical conclusion is that the Reaper "died" or was at least reduced to minimal functioning a long time ago.
- Main article: Reaper IFF
At some point, Cerberus discovered the Derelict Reaper and dispatched a research team led by a Dr. Chandana to study it. It is believed that 37 million years ago, an unknown spacefaring race fired a mass accelerator round of incredible speed and power at the Reaper, which hit and disabled it. The round continued moving, eventually impacting the planet Klendagon and creating the Great Rift Valley there. This Reaper's death was likely the unknown race's last act of defiance before their imposed extinction. Despite the massive damage the Reaper sustained, the mechanism responsible for indoctrination was still active. The Cerberus team unwittingly succumbed to the indoctrination and were converted into husks.
The Illusive Man sends Commander Shepard to the Reaper to investigate why Dr. Chandana's team has ceased communication and to search for the Reaper's Identify Friend/Foe tag, which would allow the Normandy to safely utilize the Omega 4 Relay. When Shepard's squad boards the Reaper, the ship's kinetic barriers engage, trapping them aboard. The barriers can only be disengaged by destroying the ship's core, which will also disengage the mass effect field keeping the Reaper from falling into Mnemosyne's gravity well. Fighting through the husks made from the Cerberus team, Shepard acquires the IFF and reaches the core. There, the squad encounters a lone geth, who is incapacitated by a wave of attacking husks. Shepard destroys the Reaper's core and escapes with the inactive geth to the Normandy just before the Reaper is destroyed by Mnemosyne's atmosphere.
- In one of the computer logs found throughout the derelict Reaper, a crazed Cerberus researcher refers to the Reaper as "a dead god who still dreams". This is most likely a direct reference to the famous phrase, "Ph’nglui mglw’nafh Cthulhu R’lyeh wgah’nagl fhtagn", in H.P. Lovecraft's Cthulhu Mythos, concerning the eponymous Great Old One: "In his house at R'lyeh, dead Cthulhu waits dreaming."