Tirek thought it was over when the mares he thought he had defeated used the powerful rainbow power of Harmony against him, stripping him of the power he had stolen from the ponies of Equestria, while no doubt sending him back to his cell in Tartarus. Fate, however, seemed to have other plans for him as he found himself buried under the ground, in a shallow grave that he had to pull himself out of, just as a bell tolled, and in a foreign land that he knows nothing about. At the same time he discovers that he wasn't the only one buried, as a fallen warrior also pulls herself from the grave and sets off on her own personal quest, while also finding out that they are both undead in some manner and that fate has big plans for them.
He and his unlikely companion must band together for the foreseeable future and tackle the dangers of this world, all while Tirek searches for the power that will allow him to break his condition, regain his true power, and then conquer the world, or die trying.
(Dark Souls 3/MLP Crossover)
(Part of the Demon War story series, but reading the others isn't required)