"Princess, may I speak to you in private?" Abrogail interjects. "It'll be brief." Possibly very brief; she's not sure how much Mephistopheles will even let her say.

If Celestia is surprised by this she doesn't show it. "Yes," she says. "Twilight, can you show your friends to their rooms? I've had your things brought up already."

"Yes; good night, Princess."

After another round of good-nights, everypony but Celestia and Abrogail turns and leaves the throne room.

"You told me," says Abrogail when all the other ponies are gone, "that I would be allowed to leave Equestria if I wanted."

This is a genuine surprise; Abrogail had seemed, recently, to be adapting quite well. One could almost have forgotten that she used to torture people to death for fun. Is Discord to blame?—no, Twilight and her friends used the Elements of Harmony on Abrogail; that should have removed any lingering Discordian influence.

Are the Elements of Harmony to blame? They could be.

"Yes, I did. I did not, when I took you in, agree to be your jailer—but I did promise my friend that you would stay out of Asmodeus' reach, so—there must be some limits to where you can go."

"That's going to be a problem."

"Why?"

"I doubt I can overthrow Him from out of His reach."

What??

No, seriously, what?

This would have been totally expected from Valiant Victory, who did, actually, have a convincing argument that she in particular, if not ponies in general, was being wronged by Celestia's choice to conceal the existence of the larger world and the vast amount of suffering it contains. From a pony who used to be Hell's regent on her planet—

(No, shouts Valiant Victory's voice unbidden in her mind, everypony everywhere should want Hell overthrown, and unlike you, Abrogail doesn't have the option of turning her back and pretending it doesn't exist—)

"The power that sent you to me" (who does, now that she thinks about it, remind her rather a lot of Valiant Victory) "would be impressed by that declaration, I think, but she did not, actually, tell me what to do if you made it. I'll have to—"

"No," says Abrogail. "Iomedae can't know about any of this."

"Why not? I really think, if you want to overthrow Asmodeus, Her help would be—helpful."

"I—"

—of course she can't say 'I can't tell you'; the better part of any secret is the meta-secret that the secret exists, as the Infernal proverb goes.

"I may no longer be an Asmodean, but I'm still Evil, and I still have my pride," she says. Okay, good, the mind control isn't making her put that much effort into making her lies believable. "You can—"

She can't invite Celestia to read her mind either; presumably Mephistopheles has sealed some significant fraction of her thoughts, and letting anyone notice this is forbidden.

"—do whatever you need to satisfy yourself of my sincerity."

The geas cannot cause Abrogail to devote more cunning to keeping the secret than she actually has, and Celestia is quite a bit smarter than that. It's obvious to her that Abrogail is under some unusual constraints, constraints she doesn't necessarily want to force her to violate.

"May I read your mind to determine whether you're deceiving me about wanting to overthrow Asmodeus if I commit not to use any other information I find there?"

Can she say yes? "Yes." Okay, good.

Rather a lot of her thoughts are unreadable in a way they shouldn't be for a being at Celestia's power level, but she does appear to be sincere about wanting Asmodeus overthrown.

Abrogail didn't do that to herself. There are relatively few powers who could have, and it's an obvious guess that Abrogail has somehow (clearly Equestria has a security problem, but she had already begun to realize that) compacted with one of them to replace Asmodeus. Given the emphasis on secrecy, there's a further obvious guess as to whom.

That's—

—it's not that that outcome, if as advertised, wouldn't improve the world by her values. It's just that a compact with Mephistopheles, well, isn't going to be as advertised.

It's ambiguous whether this constitutes "information she found in Abrogail's mind" but she's not, for now, going to act on it anyway, including by revealing that she's guessed. Mephistopheles must have known that she would, and she's not going to predictably discourage him from attempting this by—etc. etc. it's obvious if you understand decision theory.

"Very well," she says. "You seem to be telling the truth; I will help you as I can. Where would you like me to send you?"