The first time Ione Sala tries to cast again from a sixth-circle wizard scroll, she cannot do it, for her fingers tremble and the spellform is not unfolding well and it takes all her dexterity to avoid wasting the scroll.

This possibility has also been foreseen, by common sense more than by divine intervention.  There are other scrolls to hand, money being of no object at all anymore, and Ione Sala casts a Heroism upon herself before she continues.  It's not ordinarily what you think of as an aid to casting from scroll, but it helps if you are frightened.

And then Ione Sala speaks again.

"Magewar in ten minutes.  Enormous lethal radius around Starstone.  Start running NOW.  Avoid mage-works.  Dismiss outsiders.  Don't go ethereal.  Don't look back."

And then there's panic in the streets.

...At least among the sort of person who did not correctly foresee that they'd probably hear something roughly this alarming, update early on the evidence they'd predictably get later, and start moving away from the Starstone before the actual announcement was made.

In fact, something like four-fifths of the people managed to make that update, got their valuables and their family, shared the news with friends who didn't speak Taldan, locked up their things, and evacuated with the genuinely cautious public order of people who don't want to look stupid if in fact nothing happens.

And now there's panic in the streets, but those streets can mostly hold the least Wise seventh of the local subpopulation running around screaming.

Though even then you've got the one in twenty persons who are staying in place, or maybe even heading off to try looting a home; because they're even less Wise than that, or prouder, or just plain stupid -

"Hiiii everyyyyyone!  It's Pilar Pineda!  Sorry we had to hold the fight here, but everyone needs to run away now!  Abscond!  FLEE!"  The voice cries that command, flee, in one language after another, with a blood-freezing cheery note that is impossible to ignore.

"I, Carissa Sevar, wish to avoid deaths so numerous that the River of Souls overflows and loses some forever.  You can help by not being among them."

"I'm Keltham.  What you're about to see is a warning explosion one thousandth as powerful as the lesser forces to be unleashed here."

One ton of fuel-air explosive ignites in the air over the Starstone Cathedral, creating an enormous CRACK and a vast billow of fire.  There isn't actually any standard spell, even Meteor Shower, that creates an explosion quite that large; it looks like ninth-circle wizards getting ready to fight.

And there are, still, people so incredibly smart and skeptical that they'll stay around and try to loot a house or two.  But not enough of them to overflow the River of Souls and get eaten by astradaemons if they all die simultaneously, which is the part that matters.

Others do pray to their gods about the affair, and divine attention now gathers toward Absalom.  But there is no ancient god who decides to collect more of Themselves and focus a truly vast attention on the Starstone Cathedral and surrounding planar spaces, while nothing has yet begun for real; and that is the only choice those gods make that ends up mattering.

And then it begins.

He is about to cast a spell, from scroll, that is considerably beyond what he ought to be able to cast, even at INT 29; and it is very important that this spell go off correctly.

He draws on his familiar gloves of Use Magic Device, now improved to +7 and specialized for scrolls.

He casts from scroll upon himself, Bestow Insight:  Use Magic Device.

And he casts Prayer upon himself, from a scroll bought of a Demon Lord's worshippers.

And then so fortified, he casts Greater Heroism -

So fortified, casts Moment of Greatness -

Casts Moment of Prescience -

And at the very last, casts Wish.

In a different possibility that was foreseen, that Nethys had warned of, if things had gone according to plan, a Keltham acting with less time to prepare would have taken the shorter surer route: simply detonated a vast explosion above the Starstone Cathedral.

Keltham would (in that vision-of-possibility) have directly Wished antimatter into existence on that spot, in sufficient quantity that he could be reasonably sure that Aroden's Starstone-protecting fortress would be blasted down on the first try.

It would have been futile, then, to warn anybody but Teleporters to flee Absalom imminently.  The coastlines of nearby countries would have received that warning.

What eventuates instead is a shiver in the air of Absalom, an anti-chill, a sudden heat that isn't tangible.

Walls of force around Absalom, whether around a high-lord's palace or deep in a mage's tower, suddenly flare and shatter with shrieks of cracking magic; continual flames flicker, and some go out; a dozen different kinds of specialized magic and magical items fail, and some of those explode; a third of the mage-towers of Absalom are now in some distress; many lesser magical watchers and guardians scream and clutch at their eyes, though there is nothing visible for mortals to see; and some greater magical guardians within the Ascendant Court are charred to glowing ash where they stood.

But the real sight to see (if you are foolish enough to be looking back in that direction) is the Starstone Cathedral upon its formerly unreachable island, in the middle of a pit that nobody can cross except under their own power.

All at once the walls and roofs of the Cathedral, the pinnacles and minarets, flash into searing light, impossibly bright, as if the Starstone Cathedral stood beneath some alien sun that had exploded, though there is no explosion visible -

You obviously cannot just walk up to the Starstone by going through the Ethereal plane where it borders upon Golarion's portion of the Prime Material.  Aroden has thought of that.  An apprentice wizard would think of that.

It follows, then, that the Starstone's containing installation must have defenses that extend into the Ethereal; guessably, most of that installation just extends into the Ethereal directly.

So detonating a ten-kiloton antimatter explosion on the Ethereal plane, instead of the Prime Material, will perhaps destroy what guards the Starstone, and leave the material city standing; so you could hope.