Dearest Twilight—

Actually, no, this is too bizarre a situation to be resolved via letter.

Teleport.

(There is a flash of golden light, and a white winged unicorn twice Twilight's size—though still smaller than Katixa—appears in the library outside the entrance to Milliways. She's wearing royal regalia and her iridescent mane streams out behind her without any apparent wind.)

"Princess."

(All of the ponies jump in surprise and then bow.)

Celestia gestures for them to rise.

"I'm afraid I'll have to apologize to your new friend," she says, nodding in Katixa's direction. "Unfortunately, this portal is dangerous to Equestria and I'm going to have to close it."

Twilight looks back to Katixa. "Sorry," she says. "I wish we could have been friends. But she does have a point. It sounds like your world has a lot of problems that Equestria really isn't prepared to deal with, and it's probably better if we aren't connected."

"Okay. It was nice to meet you!"

"Actually, closing that door is the most dangerous thing you can possibly do, in the sense that the entire universe will be destroyed if you do."

Celestia sighs. "Discord, I don't have time for your games right now."

He makes an expression like a kicked puppy. "Would I ever lie to you?"

"Yes."

"Everybody out, please."

     The six ponies file out of Milliways and back into Twilight's library.

Celestia shuts the door with the nudge of a hoof.

—and Twilight and her friends find themselves, somehow, on the Milliways side of it, though it can no longer be opened by any of them.

"Um."

"Well, it appears that higher powers have decided to keep us here anyway."

Katixa looks quizzically at Discord.

"Oh, don't mind me," he says, then tears open a hole in the fabric of reality and steps through, closing it behind him.

Elorri would know what to do, or, if he did not know what to do, he would explain to Katixa afterwards what it was that made it so complicated and then she wouldn't feel so dumb about not knowing what to do.

"... you probably shouldn't, actually, mind him. That's Discord; he kind of does what he wants, and he says a lot of stuff that doesn't make any sense. Usually it turns out all right in the end."

(If Katixa has any kind of aura sight, Discord is blindingly Chaotic but not, actually, Evil.)

(Katixa has no such thing! She's a horse!)

"Anyway, we'd still love to visit your world, and since the door on our end appears to be closed, it can't hurt Equestria." Also, depending on whether it still works this way after Discord messed with it, time may or not be passing in Equestria right now at all.

"...but won't you try to go home someday?"

"Of course! I think that actually no time is passing in Equestria right now, if this place works the way it claims to, but either way a door that's only open briefly provides a lot less opportunity for evil gods to get into Equestria than a door that's open all the time. Or maybe we won't end up going home by that door at all. One of the things I want to do in your world is see if they know any other magic that might help us get home."

"Oh, they probably do! It would be expensive though. My paladin can't do it."

Twilight's familiar literally eats Wish diamonds for breakfast, but she has no way of knowing that gemstones are particularly valuable in Golarion's magic system.

"Yeah, we'll have to figure that out. Where does your door open from?"

"The church stable."

"What's a 'church'?"