((Quick aside: the reason I haven't reviewed any fanfics lately is that I have an upcoming project dealing with fanfics, and was saving those I've been reading for it. It just got delayed a few weeks, so I can review some stuff now.))
A World Without Rainbows by Uber Phoenix is a MLP:FiM fanfic. Now, I don't know if you've noticed, but I don't like Dark fics, let alone Grimdark ones. But for my aformentioned project, I needed to read another Dark fic, so I chose by far the most popular. I kinda pecked at it at first, keeping it in a tab but barely reading it. That's because what I'd heard about it wasn't what it was about at all, and it took a while for me to realize that. What you'll hear everywhere are the synopsis is that it's about an AU where Rainbow Dash didn't succeed in her Sonic Rainboom, altering the fates of the Mane 6 so that they never defeated Nightmare Moon. It is about that, but in that that universe is called Moonside, and some dimension hopping between Sunside and Moonside occurs. See, already way cooler.
On the one hoof, this fic is funny, touching, well-thought out, and incredibly well-written. On the other, Uber made Trixie turn out to be Twilight's sister. What. I've seen this several times in the fandom, and I find it just as weird as the Trixie/Twilight ship. Personally, I saw nothing that could lead to either in Trixie's episode. But whatever, at least when she's in the story Trixie acts in character. Overall, this is very highly recommended to bronies everywhere. Laters.