Almost a year after Celestia's great crusade against necromancers concluded, Mortem discovers a rather large army outside his castle. Not wanting to cause an incident, he works towards a diplomatic solution.
Rainbow wants to penetrate things and Twilight teaches her how. A few problems become immediately apparent. Too much power, too much depth, not enough material science.
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.