The greatest known enemies to ponykind were imprisoned long time ago. A thousand years. Sombra, Discord, Lord Tirek, Nightmare Moon. Great names that brought great fear. But even then, there were enemies that couldn't be imprisoned like that, that roamed free after the Elements of Harmony no longer were active. These had to be treated different, with different seals. Weaker seals. Banishments that lacked the effectiveness of the powerful ancient Elements.
One of these seals has been broken, after a hundred years somepony destroyed a barrier and allowed one of these prisoners to run free and wreak havoc once more. At least once the preparations are met.