After Jason Wright's death, the only human (or creature for that matter) known to have intentionally used some form of Interdimensional Travel to arrive thereat Equestria. 

Celestia immediately summoned Twilight to Canterlot for a meeting, but it wasn’t anything ordinary, as they were going to meet up in the underground tunnel system.

And that's when Twilight realized that maybe this was probably something bigger than she thought.

And once she came face to face with Celestia for that quick and civil chat, all she could think about was the likely stern talking she'd face, and moreover… What was it going to be about?

Only one thought wandered into her mind, a lost opportunity.

A non-canon sequel to Ending.