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.

The pinnacles and minarets are charred and charring, as the first flash fades, but they stand -

He has, a very few moments earlier, cast his first and only Wish from scroll, to conjure a significant quantity of antimatter inside of a high-vacuum space.  He did not use all that antimatter at once.  Rather, that antimatter was divided up telekinetically and allocated to magical containers.

There will be significant divine attention pointing to Absalom now, and while gods are large rather than fast they are not reliably slow.  So while he is not skipping directly to the most powerful explosion, he is not waiting six seconds between actions either.  The first explosion was more of a test than anything else.

The next Ethereal detonation is equivalent in energy to fifty thousand tons of high explosive.

A number of people who ignored Ione's emphatic instructions, and 'looked back' after the first flash, now have eyes that are scarred by the light that reflected from nowhere; and some of those will not be able to afford third-circle cleric healing.

The Starstone Cathedral is in flames, it is wounded, but it has not fallen -

Two hundred kilotons.

A wizard sobs and cradles in his arms the familiar who was burned, over and over, by some invisible force, feeling the last life depart upon some final unstoppable blow.

The Starstone Cathedral now lies blasted seemingly open -

But one cannot enter into its heart yet, nor scry it.  There's still twisted space and planar obstacles there.  Some magic like that does not pose any bar at all to material forces passing through the Ethereal, and therefore cannot be destroyed merely by nonmagical explosions in the Ethereal.

Of which, if you're familiar with a long-built-up sequence of thought about ilani weapons, there is an obvious sort of thing to try: set off an Ethereal twenty-kiloton explosion inside a jacket of raw spellsilver.  Magical forces are complicated, but some of them considered in isolation are simple.  Accelerating spellsilver sufficiently, as in, to a significant fraction of lightspeed in less than a microsecond, will produce an intense pulse of a potent magical force that interacts with other magical forces.

That is:  The adapted principle of an EMP bomb.

All spells interrupted in mid-casting fail, some explosively.

Every magic item and permanent effect in Absalom makes a Fortitude save against destruction, with DC proportional to inverse-square distance from the hypocenter.

The Arcanamirium's evacuation demiplane is cast loose of its portal; retrieving those teachers and students and servants and families will require over five hundred Plane Shifts or multiple Gates.

A reclusive wizard who lived in his tower with his secluded family, shielded from hearing any Mage's Decrees until the first ethereal blast disrupted his mage-works, was in the midst of evacuating his remaining family: a spouse, a four-year-old daughter, a two-year-old son.  His Teleport is disrupted just as it goes off, and they end up forty miles off target, downward, in the nearest liquid pocket inside that volume of Golarion's mantle.  They die very quickly, and not all to the same afterlives.

...It could seriously have been a lot worse.

In fact, it will shortly be worse.

The way to the Starstone now lies open.

One person Teleports nearby, and flies down, and makes to touch that Starstone, whose surface is now glowing hot and lightly scarred but seems still wholly intact.