Tirek has, undoubtably, thoroughly, completely, won.

He has all six former bearers of the Elements tied to his (formerly Celestia's) throne.

He has the other three alicorns trapped in Tartarus.

He's absorbed every scrap of magic from every pony in Equestria.

And nopony will ever get a cutie mark, ever again.

Three certain fillies are going to do something about that. And if they can somehow rescue their sisters in the process, all the better...

For another take on the same idea, see Crusade at Midnight Castle by Carabas; or, for a story that takes the idea in a completely different direction, see Der Unter-gang by Monokeras.