Being a ruler can be very lonely. When Celestia starts to feel that there is no one for her, one event changes her whole view. Can there really be somepony out there for her?
For most humans, being sent to Equestria and put into the body of a megalomaniacal centaur trapped in the depths of Tartarus would be the most bizarre thing that ever happened to them. For a certain knife-wielding mercenary, it was Tuesday.
A human soul is stuffed into King Sombra's body. Having no idea what's going on, and thinking he is in some kind of coma, he just casually strolls into the Crystal Empire.