So given I'm reading this fanfic for about the seventh time, and loved it every time, I figured it deserves a long, well written recommendation so that other people will read it. I'll probably repost this in other places, because more people need to read this.
The fic is
Something Blue, by
Hamhammer.
So there are a few key parts that make this fic amazing.
First and least importantly, this is a somewhat reconstruction fic, which adds little things to make the plot make more sense. For example, Shepard doesn't trust the visions just put into her head, but instead does some quick research to see if there is any evidence that Prothean relics actual do anything like this. Turns out there is, and it's written by Liara (hence why Shepard goes to save her first). Other things are even more subtle, and used to flesh out the world. For example, take the comlink. Shepard notes that she has one implanted in her ear, which was paid for by the Systems Alliance. She likes it because it is convenient, the SA likes it because it means she's always on call. This isn't just a cool factiod, but it help builds a picture of SA's and Shepard's relationship (I'll get into that later). There are countless small details like this, which enhance the story without being a distraction.
But that alone wouldn't be a reason to read this fic. I mean, it doesn't adjust the plot of Mass Effect at all, even using as much of the same dialogue as it can. So why am I recommending what seems like a Let's Play? Because it's not a Let's Play. Instead, it hijacks the plot of Mass Effect to tell a wildly different story, one about a monster and the characters around her discovering this.
You see, Shepard isn't a good person. Some characters even wonder how much of a person she is. Shepard is a sociopath, but one being blackmailed to work for the Alliance because of her biotic potential. And although the higher ups know, most of the people on the Normandy don't. But each character interacts with Shepard, and has their own reactions and path as they learn more about her. It's almost like a bunch of different mystery novels happening at the same time, as each character puts together a different set of clues. Some find out all at once (like Dr. Chakwas, who is given her medical file). Others have to slowly compile information. Some never have the chance to put it together before they die (Looking at you, Jenkins). Meanwhile, Shepard is balances her true self with a need to hide what she is, while always putting herself and what interests her first, in some casual chilling internal monologues.
Overall, this is an amazing story that I can't recommend enough.