Originally intended to be a chapter of What If... but things got VERY out of hand.
Tirek's returned. Twilight, of course, is the last pony standing against him. Or first, in this case, given that he just clawed his way out of Tartarus again.
This time, however, instead of the power of love and friendship and kindness and all that jazz, Twilight's got an orbital bombardment of anvil rain on her side.
Tirek thinks it'll be the end for the both of them. Twilight says otherwise.