Malazan Book of the Fallen crossover.
The thousandth solstice approaches. As ponies prepare for the celebrations, ancient forces begin to stir. A veteran soldier works to prepare his soldiers for the future. A deadly assassin returns to the city, bent on avenging a past betrayal. The princess of the sun looks on, detached from the world of the common pony. But behind her glowing exterior lays a dark past, with a secret that may spell her downfall.
Intriguing. I'm not familiar with the source material, so I have very little idea where this is going aside from Celestia usurped the throne and Discord was Starswirl, but I will definitely watch it.
9929958
I strongly recommend reading the original books, they are definitely one of the greatest epic fantasy series ever written. This story came around as a result of my noticing potential parallels between a major event in the books and My Little Pony. That said, I'm glad to see you like it!