Game of Thrones: Who Is Attacking the Free City of Meereen, and Why?

At the end of Game of Thrones season six's eighth episode, Daenerys is dropped off in Meereen by Drogon just in time to find the beautiful city being violently assailed by balls of fire. In episode nine, Tyrion insists to Dany that Meereen, a "city on the rise," is actually enjoying exceptional growth and rebirth. So who is trying to destroy it?

The masters, having lost their "property" when Daenerys freed their slaves, have returned to Meereen with a vengeance, intending to reclaim their absolute power within the city. As Tyrion points out, if Meereen succeeds as a city without masters or slaves, "it proves that no one needs a master." And everyone wants to feel needed, right?

The masters harbor the delusion that their little fireballs might actually intimidate the mother of dragons into surrendering the city, and as always, Dany relishes the moment when she proves that her power was grossly underestimated. Between fire-breathing dragons, an army of Dothraki, plus an army of unsullied, these fools are suddenly — and permanently — masters of none.