Toff will retreat back to a different room while his elementals open the door, scout around, and come back to report on what they find there.

The elementals don't come back.

Did his +31 Perception hear any disturbing noises?

None.

Toff casts negate aroma and silence off of a low-level utility staff that he gives a wizard minion to recharge during downtime (the silence he casts on a rock that he could drop at a moment's notice) and then flies, invisibly, towards the door he lost his elementals through.

Is it still open?

It's still open. There's nothing in particular to distinguish the dungeon beyond the door from the dungeon he's already explored, stylistically - it looks Thassilonian - but his Perception modifier is high enough to notice that the floor is tiled in pressure plates, there are holes in the wall for poisoned darts, and vents near the floor for poisonous gas. Indents on the floor and walls imply to him that the ceiling can be dropped. A hidden hinge in the floor implies that the floor can be dropped to make room for the ceiling.

There are probably more traps which he doesn't have an angle to notice, even with his bonus to Perception. And there may be traps that he didn't notice; he only barely noticed some of these. 

...That seems excessive. But, those four guys went in there and came back out, and they almost certainly didn't see the traps, so maybe they're disabled or defunct? He'll approach near enough to look through the open door with arcane sight.

The walls, floor, and ceiling of the room behind the secret door all seem to be part of one magic item, with a CL of 28.

...

Toff returns to the entrance of the Player Character Cave and casts dimension door.

He's going to bug Zenderholm about getting Salgar raised from the dead, Toff wants more minions. And he's going to ask around and see if he can find any Korvosans who knew those guys (you know the four) closely before they came walking out of that dungeon.

Zenderholm would love to raise Salgar from the dead, but she needs 15 minutes to prepare the spell and 10 to cast it, and she's been kept busy.

People who knew the four guys before they came out of the dungeon aren't super hard to find - Toff himself knew Lyvina, so there's one right there - , but Toff is unable to find anyone who knew them closely.

(In fairness, though, it really wouldn't be that odd if everyone who knew them closely is dead!)

In the mage's magnificent mansion, the hesperian devil is giving two clerics of Asmodeus a crash course on how Asmodeus sees the world.

I have a question for the hesperian devil.

The hesperian is here to answer your questions. That's why he's here. Lay it on him.

What's with the name "Rachovii"? That sounds Latin, or possibly pseudo-Latin (I don't know enough Latin to be able to tell either way).

Is Infernal a Romance language?

That does seem implied from the names that devils tend to get stuck with, yes.

But, Chelish Taldane is also a Romance language, or at least it seems that way from the names that Korvosans tend to get. So, what's up with that?

In-universe either everyone's names are different or else the Taldans based their language on Infernal or on a language that Infernal is based off of, maybe Celestial or Axisian (Axiomites in the Bestiary aren't printed as having any special language of their own, instead they know Infernal and Celestial, but I don't think that's true in-universe), or maybe Infernal and Taldane and Celestial are all based on literal Latin, because that's the kind of silly nonsense that this story gets up to. Out of universe, obviously it's that last one. Whatever the answer is, though, in-universe I probably know it but definitely can't tell you, because, like, infobudget, dude. So, can we get back to the explaining how Asmodeus thinks thing, so you can use that phylactery of faithfulness better? 

Sure.

Getting into the meat of Asmodeus's motivational structure would require getting at the nature of Asmodean Pride, Domination, Antagonism, and Spite - there are Evil gods for whom Evil is the means to an otherwise amoral end, but Asmodeus has many Evil motives -

Reebs knows this and has always known it, outside of the occasions when he's managed to forget. It'd be deeply implausible for it to be any other way, and yet Hell be what it is.

Wait, the new-minted priest who's to wear the phylactery has not always known this. He thought that the difference between Asmodeus and Zon-Kuthon was that Zon-Kuthon wants what's worst for everyone, whereas Asmodeus just wants what's best for Asmodeus (which leaves a lot more room for gains from trade)?

Uhh, I can see why people would say that, since Asmodeus isn't a straight-up utility pessimizer like Zon-Kuthon, but it's not entirely right, for a number of reasons. First of all, if Asmodeus is selfish, He's a pretty non-central case of selfishness (He more or less follows the golden rule in treating others how He'd like to be treated), and second because Asmodeus would totally enjoy pessimizing your utility if you did something that made you an appealing target in his eyes, like contest him for absolute dominion of Creation or some facet thereof (or challenge him to a game of chess or battle of wits or accept His challenge to a battle of wits or do something that He perceived as a challenge to test His wit or strength against yours even if you weren't aware that you were doing this, or, be important to someone who threw down a gauntlet such that your happiness is a component of their utility function and He can hurt them through you, because while Asmodeus is often gracious in defeat (for certain values of "gracious") He's basically never gracious in victory).

Woah, Asmodeus follows the golden rule? I didn't know He was so nice. :O

None of this matters that much, though, since, like I was trying to say earlier, it's both not really important to understand the King of Hell's motivational structure at this time (if you want to give him an easier time reading your mind, it's mostly important to understand how He comprehends sense data and evaluates plans of action - which isn't entirely separable from his goals (deopsychology doesn't work that way), but which isn't Creation's least natural thing to try and treat in isolation) and also, there's no one in the Vault who could tell you about Asmodean Pride if prompted the right way (although the humanoid with four arms that you'll be giving the other two phylacteries does understand a rarely understood part of it), so it wouldn't be any cheaper to explain that part today than it would have been yesterday.

The thing I can explain, though, is Asmodean Exploitation... without any math. Give me five minutes to put my thoughts in order. Okay.

To most beings, the default state of Creation is the state which Creation occupies, and then they extrapolate from there, based on models they've built to explain the Creation they observe, to predict the future or imagine counterfactuals. They look at Creation, ask why it is the way it is, and then ask how it could be different. Asmodeus does not do this. To Asmodeus, on a bone-deep level, the default state of Creation is the ground state - a total Worldbreaker victory - and then Asmodeus forms models to explain why this doesn't square with His sense data.

If I had to undertake the impossible task of summarizing Asmodean deopsychology in one sentence (which I do) Asmodeus is the god of creating and especially of exploiting metastable systems.