Mass Effect: Synthesis

"To the Sol Alliance and all of Humanity, greetings. I am Harbinger and I have traveled across the galaxy to meet you." Shepard wasn’t the first to reach the Catalyst, eons ago someone else did, and they chose Synthesis Though when the choice was made, Synthesis wasn’t for every living being, it was only for the Reapers and they have sought it ever since. With the Reapers now fully hybrid Synthetic-Organics beings how will the cycle change? How will the galaxy react? AU, First contact Reaper/Human, Alternate History