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.
Hades, Lord of Death and God of the Underworld, is bored. Residing in Tartarus gets tiring for a God. So he decides to visit Equestria for a bit. And maybe make new allies so he can take over Olympus.