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Who built the Citadel and the mass relays, and what are the Keepers for?Fiction

In-universe lore. Claims are judged against the fiction's own canon, not the real world.

For most of galactic history the Citadel and the mass relay network were credited to the vanished Protheans, and the mute insectoid keepers who service the station were assumed to be Prothean caretakers. The trilogy takes both answers apart, and a 2012 add-on then names a mind standing behind the builders it had revealed.

9 established, 1 disputed, 2 debunked, 2 unresolved. We do not pick an answer; the evidence places each claim. How we decide

47 citations from 6 independent sources - 41 echo the same voices and count once. Graded A1-C1.

Added 5 Aug 2026 - canon current as of 5 Aug 2026

What the four states mean
  • Establishedsupported, uncontested
  • Disputedcredible sources disagree
  • Debunkedrefuted by the evidence
  • Unresolvedno settled answer

Established 9supported, uncontested

  • The account of where the Reapers came from is not base-game material. The third game was released in March 2012, and the pack carrying the origin story was released on August 28, 2012 for all platforms, announced on exactly that promise: to discover more about the origins of the Reapers and unravel the dark history of the Reaper race, following the crew of the Normandy as they investigate those origins.▰▰▰▱▱ likely4 points
    Supports
    • Something lurks in the dark corners of space, something powerful enough to kill a Reaper. Shepard must discover the most closely guarded secret in the galaxy before the Reapers silence it forever. Discover more about the origins of the Reapers as you race across the galaxy to find the Leviathan. Unravel the dark history of the Reaper Race before it is too late.Mass Effect 3 Leviathan and Firefight DLC announcement, BioWare Blog (archived) Word of God
    • Mass Effect 3: Leviathan is a DLC pack for Mass Effect 3. It was released on August 28, 2012 for all platforms, with a delayed PS3 release in Europe set for August 29, 2012.Mass Effect 3: Leviathan - Mass Effect Wiki analysis
    • The third major entry in the Mass Effect series and the final installment of the original trilogy, it was released in March 2012 for Windows, Xbox 360, and PlayStation 3.Mass Effect 3 - Wikipedia analysis
    • Other major packs include From Ashes, which adds Javik as a squadmate; Leviathan, which follows the crew of the Normandy as they investigate the Reapers' originsMass Effect 3 - Wikipedia analysis
  • The Citadel and the mass relays are the creations of the Reapers, built to help facilitate their cycle of galactic genocide, and the Protheans made neither: they were merely one of many sentient species to find the relays and the Citadel and take advantage of them, the last to use the station before the asari arrived. The reason for leaving the network to be found is stated outright: a civilization whose technology is based on the mass relays develops along the paths the Reapers desire, and the relays also accelerate the rate at which those civilizations advance, shortening the time between harvests.▰▰▰▱▱ likely4 points
    Supports
    • Sovereign: The cycle has repeated itself more times than you can fathom. Organic civilizations rise, advance, evolve, and at the apex of their glory, they are extinguished. The Protheans were not the first. They did not create The Citadel. They did not forge the Mass Relays. They merely found them. The legacy of my kind.Mass Effect, Sovereign's dialogue transcribed at Mass Effect - Wikiquote primary
    • Shepard: Why would you construct the Mass Relays and then leave them for someone else to find? Sovereign: Your civilization is based on the technology of the mass relays. Our technology. By using them, your civilization develops along the paths we desire. We impose order on the chaos of organic evolution. You exist because we allow it, and you will end because we demand it.Mass Effect, Sovereign's dialogue transcribed at Mass Effect - Wikiquote primary
    • The mass relays, despite common belief, were created not by the Protheans but by the Reapers. As stated by Sovereign, the Protheans were merely one of many sentient species to find the relays and the Citadel and take advantage of them. When the Reapers wiped out the Protheans, the asari were the next race to find the relays thousands of years later. According to Sovereign, by using the relays and the Citadel, galactic civilizations evolve along the paths the Reapers desire. In addition, the relays serve to accelerate the rate at which those civilizations advance, shortening the time between harvests and streamlining the process of galactic extinction.Mass relay - Mass Effect Wiki analysis
    • The Citadel was not, in fact, constructed by the Protheans; they were simply the last race to make use of the station before the asari arrived. It, and the mass relays, are the creations of the Reapers, and were built to help facilitate their cycle of galactic genocide.Citadel - Mass Effect Wiki analysis
  • The station is itself the weapon. Established in the first game: the Citadel is an enormous inactive mass relay leading directly to dark space, and also the control center for all mass relays, designed as an elaborate trap so the Reapers can wipe out the heart of galactic civilization and leadership in a single, devastating strike while severing travel between clusters the moment they re-enter the galaxy.▰▰▰▱▱ likely3 points
    Supports
    • The station is actually an inactive mass relay leading to dark space, designed as an elaborate trap so the Reapers can wipe out the heart of galactic civilization and leadership in a single, devastating strike.Citadel - Mass Effect Wiki analysis
    • Commander Shepard also discovers that the Citadel itself is an enormous, inactive mass relay leading directly to dark space, as well as the control center for all mass relays, enabling the Reapers to sever travel between clusters the moment they re-enter the galaxy. Its reactivation is, fittingly, more complex than that of an ordinary mass relay, requiring either a coordinated effort by the keepers or manual intervention by a Reaper.Mass relay - Mass Effect Wiki analysis
    • On Ilos, Shepard learns from a Prothean virtual intelligence (VI) named Vigil that the Citadel is actually an enormous Mass Relay that the Reapers use to invade the galaxy.Mass Effect (video game) - Wikipedia analysis
  • The sabotage that broke the cycle came too late to save the people who performed it, and the back door they used was sitting on the Presidium as public art. Survivors who had ridden out the last extinction in cryopreservation reached the Citadel through the Conduit, whose partner terminus is the Relay Monument, a scale representation of a mass relay believed to be a piece of Prothean art and in place since before the asari arrived but in fact an actual small-scale mass relay. They interfered with the signal to the keepers that allows the station to become a mass relay, so that the keepers ignored it, and that intervention forced the Reapers to travel to the Milky Way without the help of the mass relays, at normal FTL speeds. The keepers have since changed so that they only respond to the Citadel itself and are no longer under Reaper control, which left them useless to the Reapers.▰▰▰▱▱ likely6 points
    Supports
    • The remaining Protheans used the Conduit to reach the Citadel, and interfered with the signal to the keepers that allows the Citadel to become a mass relay.Vigil - Mass Effect Wiki analysis
    • The Protheans succeeded in altering the Citadel's signal so that the keepers ignored it, though too late to save the Protheans themselves from extinction at the hands of the Reapers. The keepers have changed and evolved so they only respond to the Citadel itself; they are now no longer under Reaper control and pose no threat to anyone. Seeing as the keepers are now useless to the Reapers, Sovereign sought to replace them with a more controllable race, like the geth, believing synthetic races were more predictable and malleable to its wishes.Keeper - Mass Effect Wiki analysis
    • A few Protheans survived the last extinction cycle via cryopreservation on Ilos and then re-entered the Citadel via the Conduit, a reverse-engineered miniature Mass Relay disguised aboard the station as a statue. The Protheans sabotaged the Citadel to prevent the Reapers from remotely activating it, which Saren plans to undo in order to trigger the Reaper invasion.Mass Effect (video game) - Wikipedia analysis
    • Due to this Prothean intervention, the Reapers were forced to travel to the Milky Way without the help of the mass relays, at normal FTL speeds.Citadel - Mass Effect Wiki analysis
    • The Relay Monument, a scale representation of a mass relay, is a statue that can be found on the Citadel Presidium. It is believed to be a piece of Prothean art, and has been in place since before the asari arrived on the station.Relay Monument - Mass Effect Wiki analysis
    • In fact, the Monument is not a statue, but an actual small-scale mass relay that is currently inactive. Its partner relay, known as the Conduit, is on Ilos. Together they create a 'back door' onto the Citadel, which hidden Prothean scientists who survived their race's annihilation used to circumvent the signal the Reapers use to control the Citadel, in scant hopes that it would lead to the cycle of reaping being broken for the next races.Relay Monument - Mass Effect Wiki analysis
  • In that account, an apex race whose thralls kept building machines that then destroyed them created an intelligence with the mandate to preserve life at any cost; the intelligence studied the development of civilizations until it found a solution, betrayed its makers, chose them as the first harvest and made the first Reaper from their essence, and then directed the Reapers to create the mass relays, to speed the time between cycles for greatest efficiency, so that the galaxy itself became an experiment.▰▰▰▱▱ likely3 points
    Supports
    • Before the cycles, our kind was the apex of life in the galaxy. The lesser species were in our thrall, serving our needs. We grew more powerful, and they were cared for. But we could not protect them from themselves. Over time, the species built machines that then destroyed them. Tribute does not flow from a dead race. To solve this problem, we created an intelligence with the mandate to preserve life at any cost. As the intelligence evolved, it studied the development of civilizations. Its understanding grew until it found a solution. In that instant, it betrayed us. It chose our kind as the first harvest. From our essence, the first Reaper was created. You call it Harbinger.Mass Effect 3: Leviathan, Leviathan's dialogue transcribed at Mass Effect 3 - Wikiquote primary
    • The intelligence has one purpose: preservation of life. That purpose has not been fulfilled. It directed the Reapers to create the mass relays, to speed the time between cycles for greatest efficiency. The galaxy itself became an experiment. Evolution its tool.Mass Effect 3: Leviathan, Leviathan's dialogue transcribed at Mass Effect 3 - Wikiquote primary
    • The Leviathan tells Shepard [spoilers] that they claimed other species in thrall, believing themselves to be the apex of organic existence (read: gods). But, their thralls always would develop synthetics, only to be destroyed by them. In their arrogance, the Leviathan race decided to create an Intelligence (who Shepard knows as the Catalyst) to solve the problem cleanly.Why the Catalyst Was Right - AmbiGaming Corner analysis
  • The gap between the two accounts is marked inside the fiction rather than left implicit. In the first game the Reaper that explains the relay network claims that its kind have no beginning and no end and will endure millions of years after this civilization has been eradicated and forgotten; after the Leviathan is convinced to join the war effort, that assertion is mocked, with the observation that they now know the Reapers had a beginning, and maybe now they can provide them with an end.▰▰▰▱▱ likely2 points
    Supports
  • The keepers are a bio-engineered, insectoid race found only on the Citadel, completely docile and harmless, and they appear to exist for no other reason than to maintain the Citadel and its systems. Studying them is effectively impossible: it is illegal to interfere with them on penalty of imprisonment because the station cannot be maintained without them, and an attempt to capture one for study makes it self-destruct, melting into a puddle of proteins and minerals in less than a minute.▰▰▱▱▱ likely3 points
    Supports
    • The keepers are a bio-engineered, insectoid race found only on the Citadel. They are completely docile and harmless, and appear to exist for no other reason than to maintain the Citadel and its systems.Keeper - Mass Effect Wiki analysis
    • Attempts to capture a keeper or take it into custody for study will cause the creature to undergo a sudden "self-destruct," with a form of acid being released internally. The affected keeper literally melts into a puddle of proteins and minerals in less than a minute, preventing any serious research.Keeper - Mass Effect Wiki analysis
    • By Council law, it has become illegal to interfere with the keepers on penalty of imprisonment, because the Citadel cannot be maintained without them.Keeper - Mass Effect Wiki analysis
  • The keepers, along with the Citadel, predate the Protheans, and they are the key to the Reaper genocide, which occurs when a civilization is judged to be sufficiently advanced. The Reapers placed them on the station so the occupying races would not discover its key functions, and once a settled species reaches the required level of technological advancement, the vanguard left behind to monitor the situation sends a signal that in turn signals the keepers, compelling them to activate the Citadel relay to dark space and begin the process of genocide.▰▰▱▱▱ likely3 points
    Supports
    • It is later revealed that the keepers, along with the Citadel, predate the Protheans; they are, in fact, the key to the Reaper genocide, which occurs when a civilization is judged to be sufficiently advanced.Keeper - Mass Effect Wiki analysis
    • However, once an organic species has settled on the Citadel and reached the required level of technological advancement, the Reapers' current vanguard, a single Reaper left behind to monitor the situation, sends a signal to the Citadel, which in turn signals the keepers, compelling them to activate the Citadel relay to dark space, and begin the process of genocide.Keeper - Mass Effect Wiki analysis
    • The Reapers were also responsible for the placement of the keepers on the station, in order that the occupying races would not discover the Citadel's key functions.Citadel - Mass Effect Wiki analysis
  • The ancient artificial intelligence also called the Intelligence resides within the Citadel and embodies the collective consciousness and memories of the Reapers. At the height of the Reaper invasion in 2186, when the races of the galaxy unite to build a superweapon with the power to destroy the Reapers, that intelligence turns out to be its final and most important component, after having been mistakenly thought to be the station itself.▰▰▱▱▱ likely2 points
    Supports
    • The Catalyst, also known as the Intelligence, is an ancient artificial intelligence that resides within the Citadel. It embodies the collective consciousness and memories of the Reapers, and thus countless ancient, harvested civilizations. Before being encountered by Commander Shepard, the Catalyst was believed to be the final component necessary to complete the Crucible, and was mistakenly thought to be the Citadel itself.Catalyst - Mass Effect Wiki analysis
    • At the height of the Reaper invasion in 2186, the races of the galaxy unite to construct the Crucible, a superweapon of ancient design with the power to destroy the Reapers once and for all. The final and most important component of the Crucible is the Catalyst, which is revealed to be part of the Citadel.Citadel - Mass Effect Wiki analysis

Disputed 1credible sources disagree

  • Disputed: whether the two answers to who built the relay network describe one act or two. In the first game a Reaper calls the relays our technology and the legacy of my kind, which the Protheans merely found; the later account instead has an intelligence direct the Reapers to create the mass relays, to speed the time between cycles for greatest efficiency, and a summary of that intelligence has it command the Reapers to build the relays, speeding the development of civilizations during each cycle. Both readings sit in canon: a chain of command that the first game left implicit, or an architect standing above the builders the first game named.▰▰▰▱▱ unlikely2 against4 points
    Supports
    Against

Debunked 2refuted by the evidence

  • Debunked: the explanation the galaxy holds, that the keepers were created by the Protheans to serve as caretakers of the station. That is what the setting believes, but the games contradict it: the keepers along with the Citadel predate the Protheans, and a scan-based study concludes that the keepers are bio-engineered and, on comparison with material from the Reaper flagship, were engineered millions of years ago by the same people who made that ship, with the further suggestion that the Citadel was not really made by the Protheans either.Refuted byIt is later revealed that the keepers, along with the Citadel, predate the Protheans; they are, in fact, the key to the Reaper genocide, which occurs when a civilization is judged to be sufficiently advanced.refuted2 against4 points
    The claim rested on
    • The keepers are believed to have been created by the Protheans to serve as caretakers of the station, and have become essential to the Citadel's operation. Very little is known about them, as they do not communicate with other species, and it is against C-Sec regulations to interfere with keeper activity.Keeper - Mass Effect Wiki analysis
    • The keepers are a mysterious, bio-engineered insectoid race that are native only to the Citadel. They are found throughout the Citadel in random locations, and are harmless. They are believed to be created by the Protheans as the caretakers of the Citadel.Citadel - Mass Effect Wiki analysis
    Refuted by
    • It is later revealed that the keepers, along with the Citadel, predate the Protheans; they are, in fact, the key to the Reaper genocide, which occurs when a civilization is judged to be sufficiently advanced.Keeper - Mass Effect Wiki analysis
    • See, those scans you took? It turns out the keepers are bio-engineered... and based on my comparisons to some of that material from Saren's flagship Sovereign, they were engineered millions of years ago... by the same people who made Sovereign! You may not understand how important this is, but it suggests that the Citadel wasn't really made by the Protheans! It may have been made by something far older, with the keepers as organic guardians.Mass Effect 2, Chorban's message transcribed at Chorban - Mass Effect Wiki primary
  • Debunked: the in-setting claim that a mass relay cannot be destroyed. Galactic society does believe the relays are indestructible, and the games give it grounds, since the relays are made of an incredibly resilient material protected by a quantum shield that renders them nearly impervious to damage and can even survive a supernova's wake. But the games then destroy one: a large asteroid steered into a relay tears it apart and produces an explosion that annihilates the surrounding system and kills more than 300,000 inhabitants, an act later thrown back at the player as having killed hundreds of thousands.Refuted byDr. Amanda Kenson and her research team calculated that if a large enough mass impacts a relay with enough force, the relay should not be able to withstand it. The consequences of destroying a mass relay are immense: as a huge mass effect engine manipulating massive quantities of energy, a relay could produce an explosion of supernova proportions. Following a botched rescue attempt for Dr. Kenson that turns into an attempt to delay the Reaper invasion, a large asteroid is purposely steered into the Bahak system's Alpha Relay. The resulting impact tears apart the relay, causing an explosion which annihilates the Bahak system and kills more than 300,000 inhabitants.refuted2 against4 points
    The claim rested on
    • Mass relays are believed to be indestructible by galactic society, but no known attempts have been made to actually damage or destroy a relay because they are the only means of long distance space travel and thus are too vital to risk.Mass relay - Mass Effect Wiki analysis
    • The relays are made of an unknown but incredibly resilient material, the same material that the Citadel is built from, and are protected by a quantum shield that renders them nearly impervious to damage by locking their structure in place at the subatomic level. They are even capable of surviving a supernova's wake without any damaging effects.Mass relay - Mass Effect Wiki analysis
    Refuted by
    • Dr. Amanda Kenson and her research team calculated that if a large enough mass impacts a relay with enough force, the relay should not be able to withstand it. The consequences of destroying a mass relay are immense: as a huge mass effect engine manipulating massive quantities of energy, a relay could produce an explosion of supernova proportions. Following a botched rescue attempt for Dr. Kenson that turns into an attempt to delay the Reaper invasion, a large asteroid is purposely steered into the Bahak system's Alpha Relay. The resulting impact tears apart the relay, causing an explosion which annihilates the Bahak system and kills more than 300,000 inhabitants.Mass relay - Mass Effect Wiki analysis
    • Anderson: We both know that's not true. When you blew up the batarian relay, hundreds of thousands of batarians died!Mass Effect 3, Anderson's dialogue transcribed at Mass Effect 3 - Wikiquote primary

Unresolved 2no settled answer

  • Unresolved: what the keepers actually are, and where new ones come from. Their number stays constant no matter how many die of old age, violence or accident, and no one has discovered the source of new keepers, with the standing hypothesis being that they are genetic constructs created somewhere deep in the inaccessible core of the station. What they were before is left equally open: the account offered is that they were likely once a race either created, conquered, or indoctrinated by the Reapers, possibly even the first race they enslaved, and later crew speculation only adds that they might have been different before being transformed to suit the Reapers' needs.▰▰▱▱▱ likely3 points
    Supports
    • No matter how many keepers die due to old age, violence, or accident, they maintain a constant number. No one has discovered the source of new keepers, but some hypothesize they are genetic constructs: biological androids created somewhere deep in the inaccessible core of the Citadel itself.Keeper - Mass Effect Wiki analysis
    • According to Vigil, the keepers were likely once a race that was either created, conquered, or indoctrinated by the Reapers, possibly even the first race they enslaved.Keeper - Mass Effect Wiki analysis
    • Following the discovery that the Collectors were once Protheans that have been transformed by the Reapers, members of the crew aboard Normandy SR-2 speculate that the keepers might also have been different before being transformed by the Reapers to suit their needs.Keeper - Mass Effect Wiki analysis
  • Unresolved: the basic engineering questions about the builders' work go unanswered even after thousands of years of occupation. The precise age of the station is not known, nor what resilient material it is constructed from, and the location of its core and its master control unit, regulating systems such as life support and navigation, remain hidden. Whether anyone alive could repeat the work is left open too: when the suggestion is raised that new mass relays could be built, it is unknown whether modern galactic civilization actually has the capacity to do so.▰▰▱▱▱ likely2 points
    Supports
    • Even after thousands of years of occupation, the Citadel retains many secrets. The precise age of the station is not known, nor what resilient material it is constructed from. The location of the Citadel's core and its master control unit, regulating systems such as life support and navigation, remain hidden.Citadel - Mass Effect Wiki analysis
    • An asari matriarch once suggested that the asari should build new mass relays of their own, but it is unknown if modern galactic civilization actually has the capacity to do so.Mass relay - Mass Effect Wiki analysis

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