User blog comment:Elseweyr/Mass reading for best Effect – Introducing the Mass Effect Book Club/@comment-5619827-20140326214822/@comment-24699512-20140328103102

I think the Arca Monolith is something from a civilization before the Protheans, or, as Garhdo suggested, is even from the Leviathans, and that the Reapers have merely adopted it for their own goals.

First off, it is proven that Reapers can take control of technology that is not their own. They are extremely proficient at repurposing things for their own goals (ex: Collectors, Husks, Ravagers, etc...). Also, in Mass Effect 1, Sovereign was clearly connected with the Prothean beacon on Virmire and when Shepard used it, Sovereign immediately became aware of his/her presence since it had assumed control of the beacon. Secondly, the Reapers are not the masters of indoctrination. They are just copies of Leviathans and their indoctrination is a replica of the Leviathans' mind control, so the Reapers' methods of transforming organics into their foot soldiers, through indoctrination, dragon's teeth, and whatever other tech that is not specified, could be copied as well. Perhaps the Monolith was originally used by Leviathans to transform lesser beings into husk-like creatures, and the Reapers simply took control of it just as Sovereign had done with the Prothean beacon.

I realize that the Leviathans would not have needed technology to transform the lesser beings of the universe since they were the "apex race" and had total domination over the other species, so maybe it was an early attempt to correct the organic vs. synthetic problem gone wrong, or perhaps it was something used by a civilization predating the Protheans to achieve galactic dominance for their species. However, this is just wild speculation since there is no evidence for it.

So yeah. Old (but advanced) technology, taken over by Reapers and used to make foot soldiers.