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human
[Mass Effect Series] How would the Reapers react and deal with a race that managed to build a Dyson Sphere like the one from Star Trek Tng or the shield worlds from Halo?

I was wondering what if a race is left alone during the previous cycle but was close to space flight technology; eventually became advanced enough that they decided to build a Dyson Sphere instead of using an unknown space station with bug like janitors. In this idea this would be rather early into the cycle and the home system doesn't have its own mass relay. Ideally this race would a long lived one like the Asari.
gpt
Given how canonically dumb they are; they probably try to blow it up and kill them. This might be doable, but frankly, the reapers probably die in a fire. 

* Anyone capable of building a dyson sphere (even assuming you mean a cloud), has multiple orders of magnitude more industrial power than the reapers. 

* Anyone capable of building a dyson sphere is probably going to have all kinds of transhuman, and posthuman technology going on. Nanotech weapons seeded around relays will completely deny transit. 
Not only are they friend-foe capable, but they completely bypass shields and active defences in order to eat your hull. This sort of nanotech also probably no-sells indoctrination and whatever they use to create husks. It also rules out ground combat, unless you like super intelligent nanotech sand storms eating you alive when you try to land to gather up organics.

* Anyone with access to that much local energy will be able to produce directed energy weapons of a very potent variety. Even simple mirror focusing systems on a large enough scale are really dangerous. You're basically looking at DEWs virtually one-shotting any reapers that come within light seconds of the dyson sphere. 

* The computing power of a dyson sphere would be absurd. Even just by brute force simulating various scenarios and designs, nevermind the perceptual time advantage, the builders are likely to be as or more advanced than the reapers by the time they show up.

* Anyone that can build this doesn't need to give a fuck about relays. They can just use lasers (powered by their fucking dyson sphere) fired against pusher plates on alloy rods, to create relativistic kinetic kill vehicles. Add element zero for FTL kinetic kill vehicles. The point is, if pushing an asteroid into a relay can kill it, these guys certainly can; so the reaper strategic mobility is on borrowed time. 

Technically, in the most contrived sense, this is winnable for the reapers. By relativistic bombardment, or attempting to blitz this species and hope they didn't bother building any defences, in order to end things before the industry comes into play. But the reality is that using kiloton scale mass drivers, to shoot a dyson sphere, is a little bit like shoveling all of the snow in Siberia using a garden trowel. 

The worst part is the FTL sensors, honestly. The reason you can't defend indefinitely in reality, is that someone will eventually just throw an object at you that's moving at sufficient velocity as to be all but undetectable until right before impact. You're always trying to predictively target an object based on light that's years to minutes old, and a heavy relativistic projectile is pretty much only going to stop for another very heavy or fast object. This is obviously an extremely non-trivial problem.

But most of this problem goes away when you have faster than light sensors, which mass effect does. Suddenly, you can track this thing in real time and actually aim rather than guess. Even better, you have a dyson sphere's worth of power at your back, which makes shooting such a weapon down using energy much more practical. Since you have more time to make the intercept, you can afford to burn away at the target, or shove it off course. You actually have time to see results from plans other than a projectile on projectile hit. 

With that in mind, this is pretty close to unwinnable for the reapers as they are in canon. A dyson sphere implies a level of competence that they really lack.