(Abrogail, across the room, has noticed these events and is heading toward the scene.)

"Abrogail did it for me," she says. "And she's a more powerful unicorn than even Twilight Sparkle and she doesn't have a cutie mark either!"

"An adult blank flank?" says Diamond Tiara, squinting in disgust at Abrogail. "Ew, gross."

Quickened Hold Person. Inflict Pain.

The spell doesn't do damage or leave a mark, and the Hold will stop her from screaming. It will be a moment or two before anypony else notices that anything is wrong with Diamond Tiara.

"I do have a special talent," she whispers in her ear. "It just isn't appropriate to advertise to little ponies."

She dispels the pain after a round and walks out of the party. The Hold will expire soon enough.

My faithful student, Twilight Sparkle,

     This letter is for Abrogail. Please give it to her.

The rest of the letter is in Infernal.

While torture in the name of friendship* may be, strictly speaking, better than what you were doing in Cheliax, it has, nonetheless, no place here. That is to say: if you ever find yourself intending to deliberately inflict pain on someone, you should, instead, not. This is your only warning.

     Celestia, Princess of the Sun, etc., etc.

(*) Loanword from Equestrian. Tongues doesn't work on writing, but Abrogail can guess what it means.

May it please your majesty,

     Have you met Diamond Tiara? Every filly in Ponyville has been thanking me for "making her nice". I was even invited to join the "Cutie Mark Crusaders". They don't know how I did it, but frankly I think the balance of my actions were Good (gah!). Iomedae would have done it.*

          Abrogail Thrune II

(*) No.

The first seventeen attempts at interplanar travel without a tuning fork either did nothing or dropped them somewhere maximally uninteresting: usually either starless vacuum or the midst of an infinite ocean that Abrogail identified as probably belonging to the outer reaches of Elysium. It isn't the actual travel that's hard, really. The spell structure is just a Teleport rotated into higher dimensions; the only reason it's seventh circle for arcane casters is because, if you aren't a god, building a spell structure in five-dimensional space is impossible without some extra scaffolding. The hard part is targeting. The memory-based search component of an ordinary Teleport doesn't work at all (this was the first thing they tried), and while it would in theory be possible to Plane Shift by the five-dimensional equivalent of dead reckoning, like skilled casters can do in three dimensions with a Greater Teleport, the resulting spell would be extraordinarily difficult to stabilize and also pretty difficult to use. (See: five-dimensional dead reckoning.)

(A different ex-Chelish person might therefore have reasoned "well then let's put it in an item" and thereby made an Amulet of the Planes without ever really comprehending that she was reinventing magic lost since the Age of Legend. But she isn't here.)

Twilight's original idea of focusing the spell on a living creature instead of a piece of metal has been done before—not that anyone present knows it, but Abrogail does know Dismissal, which works on the same principle. Unfortunately, Abrogail doesn't seem to have any traces of the Prime Material left on her.

There is, of course, one idea left. Her very first, in fact.

Pinkie Pie does actually seem to have some sort of a foreign magical signature on her! It's really hard to tell where it's coming from, but there's definitely something there that shouldn't be on an ordinary earth pony.

"Spike, take a note."

"Interplanar travel experiment number eighteen, using Pinkie Pie's magical signature to target the spell."

"Spike, as usual, notify the Princess if we aren't back within fifteen minutes."

She touches the metal rod connected to the magical apparatus connected to Pinkie Pie with one hoof, and touches Twilight with the other, and—

Plane Shift.

They're in a dimly lit cave. The air is hot and filled with choking fumes, but these features of the environment are barely noticeable next to the sudden sourceless feeling of oppression—like their souls are being crushed under a thousand tons of stone.

She doesn't bother to ask how the fuck. Plane Shift out. (They did make a tuning fork for Equestria before starting these experiments.)

Nope.

Twilight collapses, staring off into the middle distance without seeming to see anything, eyes wide open and glassy.

Without even thinking she rushes to Twilight's side—Mind Blank. Planar Adaptation.

This does nothing at all.

"Whoopsie!" says Pinkie Pie, seemingly unaffected. "I didn't think we'd end up this late in the timeline!"

"Get us the fuck out of here."

And then she, too, nearly collapses as the realization hits her that she's probably been in Hell all along, Asmodeus has been having His fun, waiting for her to actually start to like friendship-and-ponies land, before revealing the truth—

Hell is the destruction of hope.

"We need to go this way!" She starts walking in the direction of slightly brighter light.