During a trip to Ponyville, to relax after a long week of work, Spitfire suddenly finds herself in an unknown world, one that seems broken and filled with conflict, along with monstrous creatures that had to be demons. From that point forward she encounters the Illidari, demon hunters that have pledged their lives to hunting down the demons and ending their burning crusade as they attempt to snuff out all forms of life that exist across the universe, only to eventually fall in with the Illidari to protect her home world from eventually becoming a target of the Burning Legion. Not long after that Rainbow Dash ends up on the same world and together they discover that Illidan Stormrage, the leader of the Illidari, has a plan to bring down the leaders of the Legion and bring an end to their foul crusade, only for fate to throw a couple of wrenches into their plans, forcing them to wait for a number of years before they can act against the Dark Titan and his forces.
Regardless of what happens to them, Spitfire and Rainbow will stop at nothing to bring down the Legion, even if it means getting involved with the forces of the Alliance and the Horde while trying to prevent another war from breaking out.
(World of Warcraft/MLP Crossover, covering both Legion and Battle for Azeroth)
(Part of the Demon War story series, but reading the others isn't required)