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)

Pinkamena was minding her own business inside Pinkie Pie's mind, only speaking whenever her other side wanted to chat or if something interesting happened, before she was ripped out of the life she was used to living and was dropped in an entirely new world, with a body of her own no less. Due to some injuries sustained in her fall she has a fateful encounter with someone who she later learns goes by the name 'Muzan', an event that changes her life once again and sends her on a quest to save this world in her own way, all while seeking to find the figure that had changed her life and learn more about him.

(Kimetsu no Yaiba (Demon Slayer)/MLP Crossover)
(Part of the Demon War story series, but reading the others isn't required)

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)

Sunset Shimmer almost succeeded in taking over Canterlot High, stopped the Sirens from taking over her new home, and managed to prevent the destruction of two worlds. She feels that she deserves a break and decides to return to Equestria during her summer break. Unfortunately, when she enters the portal she appears in a land that is different from the worlds she's already been to. Now she has to save this world before she can even focus on returning home.

Unfortunately, she's not the only Equestrian that's appeared in this new world, as one of Equestria's most dangerous villains is loose to do whatever he pleases. How will she cope with having to save this new world from Tirek?
(This is a different Skyrim story than my Sombra visits Skyrim story)
(Part of the Universe 13 story series, though reading the other stories is not required)

Lyra passes out on her way home one night, but when she wakes up she finds herself in a strange realm of fire, ash, and creatures that she could only refer to as demons. Before she meets what she assumes is her end, she is saved by a group of bipedal creatures that take her to their master, someone named Illidan Stormrage. There she learns what the bipedal creatures are preparing for and is offered a choice; either join them or return to her home.

The problem, she discovers, is that there is another displaced pony wandering the land of Azeroth; so she has to choose between a rock and a hard place. She only hopes that she doesn't regret her choice after making it.

(World of Warcraft; Legion crossover with MLP)
(Part of the Universe 13 story series, though reading the other stories is not required)