Many ponies believe that Tirek was only a story, the last of his kind. They're wrong. There is only two centaurs left, trapped right next to each other in Tartarus. If his magic is the cost of freedom, he's willing to sacrifice it to Tirek.
Set right after Twilight becomes OP, Fluttershy and Rainbow Dash are excited for a famous pegasus stallion, a great weather colt who has come to help Ponyville with rogue Everfree clouds rolling into town.