Probably one of the most immersive and well thought out AUs I've ever read. The plot, characters, and relationships are all pretty great, too, with each character feeling true to their canon counterpart, even if their circumstances have altered them substantially.
An immortal Salazar Slytherin rules the wizarding world with an iron fist. Magical society is split up into strict castes, with the Serpent caste at the top, and Mudbloods as casteless cannon fodder in the ongoing war against Dark creatures. Dumbledore and the Order of the Phoenix are outlaws and revolutionaries, while an embittered Tom Riddle is the Headmaster of Hogwarts, with the Death Eaters as groundskeepers and guards.. Even Quidditch have been swapped out with War Games. It's a total AU, where almost nothing is the way you're used to seeing the Potterverse. It's darker, more war-like and more oppressive. You get the familiar characters, who are mostly familiar from canon, but most of them have been put into new roles, and some of them have either subtly or completely different characterizations thanks to having grown up in a very different world. This sounds like it's a recipe for disaster, but it's actually REALLY GOOD. The writing's top-notch, the dialogue is great, and the darker tone is balanced out with many moments of levity. And some of the alternate roles for the characters are really inspired... Gilderoy Lockhart as a radio personality, for example.
This fic earns a five star rating for being just on the right side of horrifying and excellent world building.