You get a machine implanted in your spine that looks like a blue beetle. You can remove this from yourself and put it back, it just hurts a lot. This machine has an onboard AI, which, while somewhat sociopathic and inhuman, is loyal to you.
As for what abilities you get from this, starting out this is capable of self-improvement, is capable of building objects and shifting shape through nanotechnology, has highly advanced scanners capable of scanning for life through buildings, can translate any earthly language and project holographic images, can form the expected suit of powered armor over yourself to grant superhuman physical abilities and flight, and can create advanced sonic and plasma weaponry capable of harming other Reach Scarab users at this same level.
It should go without being said, but this doesn't have any sort of failsafes aimed to subvert the Scarab against the user like how the Reach typically produces this technology.