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, embedding the desire in the Reapers. They've searched for organics to merge with ever since and now they've found them. AU, First Contact, Humans / Reapers