Twilight and Spike both went on a trip.
A trip to the same place that houses all of Equestria’s villains.
After opening (and closing) the big, double doors to the prison, and getting passed Cerberus, the two soon find themselves at the cell that currently holds Tirek.
“Well, if it isn’t the "princess of friendship" herself. Back so soon?” Tirek deviously said just at the sight of the two.
“Tirek.” Twilight said sinisterly.
“Cozy Glow!” Cozy said cheerfully.
“Yeah, remind me again why Cozy’s locked up in here again.” Spike asks quietly to himself, scratching his head in confusion.
“So, why are you here? Here to gloat about defeating my plan without you actually doing anything?” Tirek said jokingly.
“No, as a matter of fact. We are here to ask if you know, anything, about another centaur, like yourself, called Galbus.”
Tirek, after hearing the name Twilight just spoken, looks at the two in surprise, then starts to sweat nervously. “Why, what do you wanna know?”
Twilight levitates out a map of Wysteria’s field. “‘Cause it appears that he’s back. And that he wants to ruin the lives of everypony I know-”
“Oh nononono. You must be mistaken. There is no defeating him. Only banishment!"
“Really? And how do you know?” Twilight asked
“Oh, it’s nothing. But you must leave this place, NOW!”
Right as Twilight’s about to speak, Spike coughs up a letter from Princess Celestia, most likely.
Twilight takes the scroll, opening it up and reading aloud: “The Galbus has returned, and it’s in Ponyville, right now.”
Twilight immediately drops the scroll and starts running back towards the door, Spike doing the same, though trying to catch up more like it.
As both Twilight and Spike exit Tartarus, Tirek, watching the two leave, says to himself: “Galbus. That’s a name I’ve haven’t heard in a long while.”