King Sombra has returned and snatched the Crystal Empire in Princess Cadence and the rest of Equestria prepares for his next play to take more, but it never comes. And after the first few escapees from the empire, no other refugees flee to Canterlot or other cities. The princesses are very, very confused, but have to reclaim the Crystal Empire either way.

Sombra takes a little while to show up directly, but the story really is about him so I didn't think anyone else got enough focus to tag them.