So what does dath ilan think actually happened?

Lacking direct observation, you would probably have too many hypotheses and too little evidence to narrow them down.

You might speculate that the currently-known Starstone was the whole original 'poisonous remnant' rather than just a fragment.  If there were fragments, they might have been from pieces of moon, blown outward along a similar trajectory after the original Starstone blasted straight through the moon at high velocity.

You could theorize that the Starstone was aimed from the beginning to pierce through Golarion's moon toward Golarion - rather than Acavna moving the moon into place, and in futile error - and that the Starstone killed Acavna through Her connection to that moon.  Possibly Amaznen tried to save His sister-deity and the Starstone sucked the power out of Him and killed Him too.

This hardly seems certain.

Yes yes, it's admitted speculation; but it's often wise to start by trying to develop any consistent speculative model at all, if you're trained and confident in your own ability to throw out any pieces or wholes in which you later spot a problem.

Mmhm.  Well, keep speculating, then.

Maybe the alghollthus tried to catalyze the Starstone's 'poison' with their magic rather than having the power to create a weapon like that from scratch.

Or maybe, the alghollthus had nothing to do with the entire matter and were only blamed afterwards.

...Or maybe it wasn't the whole alghollthu civilization, but a particular band of alghollthu mages influenced by Rovagug, like how Rovagug is said to have infected a Sarenrite city built far above the Dead Vault.

Why blame Rovagug?

Abadar was taken by surprise at how the Age of Darkness ended Zon-Kuthon's exile, so some prophecy-breaking force was involved at some point.  And if Amaznen and Acavna did willingly sacrifice themselves, maybe it wasn't to save mortal lives; maybe it had more to do with Golarion being Rovagug's cage.

That in turn suggests that an unmeliorated Starstone strike would have benefited Rovagug, or that the gods feared it would.  Maybe the missile was 'accidentally' aimed at the Dead Vault?

...But the main point is just that the thing that is written in most books, can't possibly be the true story; and that's obvious as soon as you try to visualize the process step-by-step.

And you might also suspect, if what was written in books seemed obviously wrong, that there was some reason the real answer was hidden - though the general state of disrepair in Golarion epistemology, from the standpoint of the isekaied physicalist, is such that even this guess is very tentative!  The myth-composers could just be that invalid.  But still, you'll go looking for truths that are not only overlooked but concealed within the Starstone, faced with a story like that.

It begins in the City of Lost Omens, with a casting of the spell Mage's Decree, by which a wizard can make themselves heard for miles around, all through a city...

"This is Ione Sala, oracle of Nethys, apprentice to Nefreti Clepati.  Stand by for extremely urgent instructions in three hours; prepare to convey or translate those immediately."

A lot of predictable ruckus follows!  Those who didn't understand the original words (in the language there named Taldan, called also Common in Absalom) shout for translations.  People who've lived their life well enough to stay ignorant of wild rumors are even more confused, and inquire after context.

Many wise and proactive people will panic early, to avoid any later rush; and start heading away from the Starstone, since earlier rumor held that's where shit would go down.

Certain others, however -

“- that’s illegal, eighty thousand pounds in fines, she can bargain it down if she talks to the Primarch,” says Cassdra Eliomole instantly. The nice thing about violations of this specific city statute is that you can’t really contest that they happened. (Guilty parties can of course teleport out, but it’s the rare high-level wizard who will accept being permanently blacklisted from Absalom’s shops and libraries and mage academies.)

The nice thing about this specific violation is that the wizard even named themselves, unless they are naming their ex-girlfriend to get her in trouble, which is the sort of thing wizards frequently come up with, thinking themselves cleverer than they are.

  “- do you suppose it’s true, though,” says Mesich Aspexxon, her secretary, who has cooperatively gotten out the paperwork to be delivered to Sala issuing the fine and explaining how she can contest it, and is now filling it out with today’s date, the time, the investigating authority, the statute violated, and the source of the information by which Sala was identified as a suspect (“testimony to all Absalom”).

“Don’t be ridiculous.”

  “I mean, there’s the prophecy, right, and Ione Sala’s that Chelish Nethysian. She might know if the prophecy’s true, and the city to be devoured.”

“You can’t trust a Chelish person, and you can’t trust a Nethysian,” says Cassdra, with absolute surety. “So any sound a Chelish Nethyisan makes is just so much wind whistling through the streets, and we’re going to fine her for it.”

  “Well at minimum,” says Mesich, “I think we’ve got to think of who’s delivering the notice, you know, if she’s in a fighting mood, considering that you can’t trust Chelish Nethysians.”

…that is a complication. Cassdra bites the end of her pen.

The Office of The Public Interest, which handles the frequent and often impressive misconduct gotten up to by the high-level adventurers of Absalom – anything not appropriately handled by the city watch and the option of dumping some idiot in the drunk tank or the gaol – is the province of the First Spell Lord of Absalom, who is responsible for regulating the Arcanamirium and ultimately all spells cast within the city.  However, the position of First Spell Lord is currently held by Lord Gyr, of House Gixx, the Primarch of Absalom, who's not in fact a caster, but assigned himself that position anyways.  He's not in Absalom right this second.

The next most obvious person to storm in, given the implied threat to Absalom, is the Second Spell Lord charged with Absalom's magical defense:  Lady Darchana, of House Madinani, who also happens to be archdean of the Arcanamirium (though at seventh-circle she's not actually the most powerful arcanist there).  But Lady Darchana has long been on fragile terms with Primarch Gyr, both as his obvious rival for the true mastery of Absalom, and also about the snub of her not being named First Spell Lord.

    Mesich finishes the paperwork. “- Darchana?” he asks quietly.

“No, we can’t,” says Cassdra. “Gyr will rightly consider that entirely inappropriate - and think, she might declare that this represents a real threat to the city, which she of course will vanquish in his absence -”

      “Then you’ve got to contact him!”

“I am doing that!” Cassdra is applying his seal to her incident report at this very moment. Gyr resents, and is usually shielded against, Sendings, but he has some astoundingly fast carrier pigeons. He’ll hear of the problem in plenty of time.

—----

“I should really do something,” says the Lady Darchana, contemplatively, to her familiar, a specialty pug named Tiffany. “Why, the girl implies a threat to the whole city exists, and I can’t imagine the Primarch competent to handle it alone!”

  “And yet,” Tiffany says, “one imagines he will hesitate to ask the Arcanamirium for the help he so obviously requires of it.”

“Well, he’s going to have to bestir himself to request our aid sooner, rather than later.” She fixes her hat in her reflection in her bedroom window. Through it there’s a courtyard visible. The wizards of the Arcanamirium are, well, Teleporting. Out. It’s not that they won’t come to Absalom’s defense if necessary, but one can come to Absalom’s defense just as well from several hundred miles away, and, well, there is the prophecy, and even if one puts no store in the prophecy there are panicked people who do and will suddenly pay very well for a ride out. “I heard he’s out of town.”

  “He has that house in the country,” Tiffany agrees cheerfully.

“You know, I think it speaks very poorly of him, that he didn’t delegate the defense of the city to me in his absence, in light of recent events.”

A servant timidly sticks her head around a corner. “Yes?” says the Lady, graciously.

    “My lady, are we - leaving?”

“Us? Why, no. The Primarch may leave the city to incompetents while he vacations in the country, but the Arcanamirium remembers its duties, and doesn’t fear them. Why, I’m going to go apprehend that troublemaking Sala myself, as soon as the Primarch asks it of me.

There’s a demiplane we evacuate to in the case of threats to the city, with an access point under the wine cellars. You can get your family, if you’d like, so long as it’s clear you’re being very silly and that I’m not perturbed in the slightest. ”

    “Yes, my lady.”

“And so long as you are sure to repeat everything you overheard about the Primarch.”

    “ - yes, my lady.”

Cassdra presses her paperwork on the process-server for Public Interest, once the carrier pigeon has taken wing. “Do go and serve her notice at once, if she’s still in the city. I’ve raised the fine to three hundred thousand pounds, on the grounds that she caused mass panic and probably dozens of deaths of trampling down by the docks.”

  “You can take the Asmodeanism out of them, but not the Evil,” Mesich says gravely.

He shouldn’t’ve said it in front of the notice-server, who is blinking very rapidly now. “She’s not a murderer,” Cassdra says impatiently. “She’s not even a renowned adventurer. She’s a self-important vandal with one sixth-circle scroll. A dime a dozen, really. By the time the Primarch sees her she’ll be very apologetic, I don’t doubt, and in any event all you’ve got to do is serve the notice.”

Ione Sala can be located readily enough, if Office of Public Interest has magic that can do that, or if they correctly guess Iona Sala would be near the Ascendant Court and ask around if anyone's seen her.

The young woman in question can be found grimly vanishing away whole bookshelves of books from a library in Iomedae's Temple in the Ascendant Court near the Starstone Cathedral.  Ione Sala reads Neutral Good, and she's sworn an oath to Nethys to give back all the books that she's now taking into her protection.

Acolytes are rushing about her, in the way of people who are trying to get everything valuable out of a temple.  Priests and paladins are nowhere to be seen.

Ione Sala will accept the fine notice without blinking, fish a correspondingly large diamond out of her pocket, hand it over, and tell this minor functionary that she's extremely busy and he needs to get out of everyone's way NOW.

He departs as rapidly as possible, thinking uncomplimentary but not false things about wizards, Nethysians, and Chelish people, desperately trying not to lose the diamond or be observably in possession of the diamond.

"I think we ought to give due consideration to the theory this has something to do with the prophecy," Afsel Kellaris says, in the emergency meeting convened to discuss Ione Sala's actions.

"I was worried someone was going to waste our time with that," Jepata Eres, his immediate superior, says. "Of course it's related. Sala made it up for attention a while ago, and is escalating now. But the first wasn't true prophecy, which every schoolchild knows is broken, and the new one isn't either."

"It could still be, you know. Not prophecy. Prediction."

"But why, I ask, didn't you say 'prediction'? Because it doesn't sound as dramatic as prophecy. Ione Sala predicted and then caused panic and mass death in our city. That's the problem we're facing, and if it were up to me she'd hang for it four or five times."

"The thing I was trying to say was, maybe she knows of, I don't know, some unsealed ancient horror -"

"If she knew anything real and were actually interested in helping us prevent it she'd tell us what she knew. She's an adversary; she doesn't care about this city; nothing she says is trustworthy; nothing she claims merits consideration." Jepata's tone is one of finality. "The thing killing people out there isn't prophecy, it's panic. We need to get the city guard into the streets, impose a curfew, keep people home, or we'll have another trampling incident worse than the Steertrack Races -"

"With all due respect," a man objects from across the room, "you're not going to like desertion rates if you order the guard to their posts right now."

"What's your suggestion, I overlook the desertion and hope they sheepishly return to their posts tomorrow, and let the people trample each other into the bloody mud?"

"No, but you've got to reassure them first! People need to know that we looked into Sala's claims and there's nothing to worry about! We need to get our own counter-message out there, telling people that we've stepped up monitoring of threats to the city and there are no signs of any problems -"

"Oh, are there not?" someone else interjects, relieved -

" - well, I haven't heard of any, and I'm sure it'd have been shouted to the rooftops if there were -"

"You can't ask the Guard to go back to their posts while they're terrified for the lives of their loved ones. You've got to assure them that there's nothing to worry about."

"Yes," Jepata snaps, "someone already said that. I'm working on it. A message to all city watch - the rumors have been examined, they are false, there are no risks to the city, no one in city leadership is departing -"

"Departing is dangerous!" offers Afsel, reorienting to the new priorities. "The ships might be sunk, Sala might be feeding people into the mouths of pirates -"

"Excellent, yes," Jepata agrees. "Departing puts your family in grave danger, what's safe is keeping them home, Sala's claims have been decisively refuted - yes?"

"Yes," everyone agrees.

"The best way for the populace to be safe from the panic is to remain in their homes.  We'll send more of the city guard to the Ascendant Court -"

"I'd say it's the Docks district that needs 'em," offers somebody who is not at all suspicious and is totally supposed to be inside this room.  "Some people are fleeing the Ascendant Court, but that's their own foolishness and it'd serve them right to return and find their homes robbed.  It's the docks where there's fighting, fighting that might spread -"

It takes them half an hour to draft and authorize an announcement using Mage's Decree, to counter Sala's absurd misbehavior and assure the people of the city that all is well. Most of that time is spent quibbling over wording. They don't get many words, and a number of people want a say in what they are. By then, the city guard has been largely deployed to the docks to manage the masses of people desperately trying to board boats; the rumors that are flying assert that everything from Zon-Kuthon to returning Aroden will be showing up, or maybe that the island will be swallowed into Hell.

But that's not to say that it's too late for the announcement. There are many homes whose residents are bitterly fighting over whether to flee or to stay, many people trying to persuade an entrenched relative or agonizing over whether to spend their entire fortune on a Teleport, many people nervously peeking out into the streets and taking the temperature of things. Many people who want to hear from their city government and know what they should do.

The statement eventually agreed upon is, "Remain in your homes. Sala's claims have been examined and disproven. No danger exists. All deaths have come from panic. Protect your families: stay home."

The minute it is approved, it is passed on to a wizard who was able to prepare the Mage's Decree spell in the last half-hour.

As the wizard is in the middle of casting, his spell is disrupted by a pineapple pie being smushed into his face.

The Arcanimirium has a higher percentage of fifth circle and above wizards of any wizarding school on the face of Golarion, but that's still only around ten percent; with all of them out selling Teleports or fleeing the country it barely feels less crowded.

No one has dared asked the Lady Darchana what she plans, but people are hovering lest she happen to say it offhandedly.

She's not an idiot. Nethys is playing at something, and Nethys likes big explosions, so whatever Nethys is playing at is plausibly catastrophic. Warning everyone and then doing a big explosion would be in character. The assurances she's heard from the city government are meaningless.

Of course, reacting strongly to warnings of catastrophe is a great way to be easy to manipulate, and to be on the wrong foot for catastrophes coming in from an unexpected direction.

     "My lady?"

"You fear that our deaths are coming and want me to order an evacuation but are perfectly aware it is as likely as not to serve our enemies," she responds without even looking who spoke. "You can stop asking. I don't know what I'll do, but none of you have useful input."

      "Pineda's been spotted. Interfering with the city in their efforts to reduce panic and get everyone to stay home."

"The whole nation of Cheliax should have long since been burned off the face of this benighted world," Lady Darchana says with feeling.

       "Does that mean we're not evacuating? Since they want us to?"

"Would you shut up?"

She is surprised, and dismayed, and even disappointed, when Her priests in Her temple in Absalom ask Her for guidance about the Nethysian demanding that the temple be evacuated. She makes the obvious inferences: Some persons adjacent to Keltham, and maybe Keltham himself, are going to go for the Starstone.

This is good news, because the Starstone isn’t Rovagug. It’s bad news, in that the Starstone also is not ‘Keltham goes to Lastwall, and meets people whose convictions are like his but have settled around them into a shape humans can endure’, and in that Keltham is reasonably likely to die attempting this.  She alone would not be capable of saving him, even if She would, nor does it seem likely that Cayden would be sufficient - but perhaps She’s underestimating Him, or underestimating Keltham. She has seen Keltham in vision; whatever Keltham is doing here, he expects it to succeed despite all obstacles that he knows about. It may still be hugely ill-advised, or worse, intelligently contrary to Her purposes. But Her fragment's guess is that Keltham's purpose is unlikely to be worth expending Her very scarce resources to prevent.

She tells Her people to evacuate civilians, since they’ve gone and conveniently made this a single bit of information to transmit to them and they can probably save enough lives to make it worth it.

And She pays the situation more attention, and watches over it for a time; but in the end She decides not to draw together enough of Herself to participate in a fight when someone touches the Starstone -