Shut up, dath ilan.

It doesn't go quickly, with only one Summon Monster VI to use per day.

Esta doesn't remember, without a reference handy, everything you can get with Summon Monster up to VI.  And he can't, in fact, get a copy of a devil -- not for any key special ability, but to write for Esta the full list of Summonables; as is the sort of clever strategy that Cultural Adaptation (dath ilan) immediately suggests you would say to a twelve-year-old you were showing how to Think of Things.

He can get an Earth Elemental.  But the Earth Elemental seems to have trouble understanding the question.  After three failed rephrasings, Esta gives up and at dath ilan's suggestion tells the Earth Elemental to look through the nearby stone for gold or spellsilver or diamond, in case such materials are much more common on entirely untouched planets or just this planet in particular; but none of those can the Earth Elemental find or retrieve in just a half-dozen rounds.

And when Esta succeeds in summoning a Lawful (Evil) Kyton from the Shadow Plane which purports to know, and obediently writes onto stone with a sharp finger, everything callable by Summon Monster VI -- some of the monsters the Kyton names are entirely unfamiliar to him, and it is missing staples like Dire Bear and Triceratops that Esta knows are on the list.  For, it seems, what creatures it becomes possible to Summon, varies by what planet you are on...

...and Golarion is perhaps unusually restricted, about that?  Esta has never heard before of a 'Sahkil', as in the entire Lawful Evil order of corrupted psychopomps that apparently lives in the Ethereal.  Let alone a 'Pakalchi Sahkil'.  Though a few questions asked of the summoned Kyton make it clear why Golarion might be better off for not being able to summon such a thing; they feed on broken relationships, and consider healthy Evil relationships just as tasty to shatter as Good ones.

But a Pakalchi is one of those rare kind of outsiders that Summon Monster VI can copy, apparently, if you're on a deserted planet and there aren't many local restrictions and specializations on the spell you get.

And if Pakalchi live in the Ethereal, then they should be summonable even in this place, according to Esta's current guess about the rules.

...you know, Esta is going to try his / dath ilan's bright idea with a Xill first.  Maybe it just works with a Xill and there won't be any need for a Pakalchi, or whatever the Abyss a 'Hound of Tindalos' might be.

Estha seems to be throwing more effort at their problem! And he's even telling her some things! He informed her that he was intending to [summon] an entity from [Ethereal]-adjacent planes, and then, deadpan, that if anything went wrong it would be her fault for telling him to think of things, which is such normal and characteristic Estha-teasing that, again, it left her feeling a disorienting mix of reassured and faintly unsettled. 

...She's happy to either come out with him, if there's any way she can be helpful, or else stay back in the new shelter in case he needs to Word of Recall back. It remains the case that most things that could go wrong will not instantly incapacitate him such that he can't cast it at all, though she'd like to send him with the radio locator-beacon just in case so she has a way to go looking for him if he doesn't report back. 

Probably better that Merrin stay here; there's not anything obvious she can do to help with this part of the operation, and her presence would be one more added variable, even if the [summoned] entity is theoretically supposed to unhesitatingly obey all instructions and not act otherwise.

And Esta makes sure that all of his magical items are about his own person -- it ought to be hard to miss, but he does a Detect Magic just to be sure and oh right that's a Continual Flame skullcap not actually just a room light, he needs that too.

And Esta heads over toward where the tidal bore first hit him, with every single magical object about this planet now attached to his own person, except one.

And then he is standing in just the place where he first arrived to this exoplanet.  For luck, maybe, or symmetry, or in case the planar boundaries are any more permeable in this exact spot.

Summon Monster VI:  Xill.

Xill can planeswalk, to the Ethereal and back.

But summoned creatures cannot use teleportation or planeshifting abilities.  Certainly not for passengers.

But some of them can apparate small distances, if that is one of their abilities, on the order of a Dimension Door.

Esta genuinely doesn't know if this will work.  Well, if it doesn't, it is dath ilan's fault.

"Look into and through the Border Ethereal, from where we stand," Esta speaks in Infernal.  "Do you see the openings into any extradimensional spaces, anywhere around us, for as far as you can see?"

The Xill swivels its head, up, down, side to side, all around in every direction.

Finally it rasps, in Infernal, "No."

"Was that command one you were competent to follow in the first place?  Would you have expected to see a Bag of Holding, if it existed?"

"Perhaps."

"Look in every direction around you through the Border Ethereal, piercing all about this planet that is merely material.  Do you see anything magical, anything unusual in any way, that is not one of the magic items immediately around my own person?"

Again the head swivels about in all directions, and then at last the creature rasps, "Yes."

Esta realizes, then, that he had not expected and had not really considered that this might actually work.

"What direction and distance?"

The creature points along the rough path of the tidal bore, with a single claw.  "I know not the exact distance," it rasps in Infernal.  "Perhaps one and a half dozen dozen dozen feet."

That is territory they've gone over with Locate Object!  Not often, because that would be stupid, but certainly more than once!

Esta doesn't get it.

Did he lose some other magic item that was about his person?  That he is somehow not remembering?

The angle of the claw is not pointing right for the object to be far below the ground, as was one of Esta's worries.

"Can you see, and would you see, if the object of interest is obscured behind lead or some other shielding material, somehow?"

"I would not see."

He tries to think, then, as dath ilan would have him think, but he cannot think of anything before twelve moments have finished passing.

He marks then the bearing that was given him, by scraping into the ground with a metal edge of a piece of his armor.  They can triangulate; or they can try again closer to the given direction with another Xill, to get a clearer direction and a shorter distance.

But first, Esta will look around that rough area, fucking harder this time, with Detect Magic and not just Locate Object, in case he is somehow looking for something that is not his Bag at all.  For the volume that they had thoroughly scoured with Locate Object, they had not rechecked all that intensively with Detect Magic.

In retrospect, there is a different way that people have, of looking through a place where something might be, and looking harder through a place they have reason to believe contains the fucking prize.

Wind Walk was the spell that Esta traded away for Summon Monster VI.

But he has gotten back Air Walk, and he still has Ride the Waves.

Esta will not be much impeded as he walks, half-runs, through the endless road that the air has become for the next 2 hours, until he gets to the place that was pointed out to him.

Detect fucking Magic.  He doesn't bother with Locate Object, it already hasn't worked for whatever reason, and maybe he's not here to find the Bag at all.

He sticks his head into crevices, he tries to overturn small rocks, he looks around corners, he goes into caves and scours their waters and then airwalks up to the ceilings to check for rock-concealed crevices and crannies there.