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.