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)