You may call us Reapers. You may call us the end of all life in the Galaxy. You may call us the ironic, robotic perpetrators of the endless cycle of extinction all in the name of preventing a Robot war with a Robot war. But we have a purpose.
Which purpose? Look around yourself. Did you ever noticed that race called "Humans?" How that, whereever you go, you are bound to find them around you, even in completely inappropriate places? Thessia. Illium. Daikira Ward. Tuchenka. Omega. Bekenstein. Noveria. Palaven. Virmire. Racial homeworlds. Quarantined worlds. Asteroids. Secret facilities. Citadel colonies. Places where other races weren't supposed to go.
Only Human colonies allow other races to land on them. And Humans are always found accompanied by other races. They have no distinct faction, instead tag along other races and does nothing politically. They have patchy official records and the history of one can never be fully elucidated. Have you ever understand why?
Short answer: Humans are an anomaly. Both uncontainable and antimemetic. An errant quantum field which Planck's constant mismatch with the rest of the universe. A "fifth force" within the universe if you so wish.
Long answer: Humans are far, far more ancient than what they want you to believe. Have you ever seen the tablets unearthed on Namakli, a primitive depiction of the Leviathans? Well, the bipedal race depicted below were Humans. And the Humans predate even the Leviathans. They predate our creators. When the Leviathans first came out of the oceans they evolved in, the Humans were there. The Leviathans attempted to enthrall them to use as workhorses, which kinda worked. Then the Leviathans found out that the more they use the Humans, the more common the Humans become. The leviathans are eventually outcompeted, their vast colonies overrun by the ever-increasing quantum field. Facing extinction, the Leviathans retreated back beneath the oceans on their own planet. Where they went to an extensive period of research, finally coming to a conclusion: it was the Leviathan's own success that caused the Humans to grow unbounded and take their place. You see, the phenomenon known as Humans were composed of two different components that are the different manifestations of the same thing: A quantum field which they manifest from and demanifest into, and the individual Humans themselves that manifest from and demanisfest into the field. A form of Wave-Particle Duality. And it just so happened that the mechanism of which cause the wave-function collapse and the manifestation of individual Humans from the field was the presence of sentient beings themselves. The more Leviathans there are the more Humans pop out from the field due to the observation from the Leviathans themselves. Shooting them does not work because dead Humans return to the quantum field, as does any Human that happened to eat or pick up material from our universe before demanifesting which takes the extra mass to the field, strengthening it gram by gram until the entire area is overrun and consumed. So we were born. The Leviathans created the Catalyst, tasked the AI with the explicit task of reducing the amount of sentient observers within the Galaxy to as low as possible, hoping that the field will dissipate if we cut off the source of their energy/mass. 50000 years were considered a mark, We were created as the enforcers of the cycle, and to check the Galaxy for the signs of sentience and the presence of Humans every 50000 years after the last cycle of sterilization. If Humans were found, then the harvest of this cycle begins.
We have already accumulated hundreds of thousands of cycle-worth research data. From every sentient race we have harvested during that cycle, and from every bit and piece of research we have accumulated. We have devised a plan. A device that would permanently usurp one of the quantum fields of the universe. Ending the Human infestation once and for all. However, Our creators were already extinct. From their long self-isolation, both from the Humans and from our young selves when the Catalyst decided that the Leviathans themselves were sentient observers that need to be eliminated in order to stop the Human infestation from spreading. We are now left without a purpose, and it's for you to choose.
You can either finish what the Leviathans and the Catalyst failed to do, and eliminate the Human quantum field from the universe, or if you think that the Leviathan's wills and wishes should best be left for history, you can use the same device to eliminate our own quantum field- Putting the haunting remains of the Leviathans rightfully to it's grave. Either way, there are no more decisions the Catalyst could make as the Leviathans were gone, and all the choice are now yours.
Make your choice. Player.