As her primary task, Keltham sets Carissa to crafting a +6/+6 Intelligence/Wisdom headband for himself.

Splendour would be improbable, as something that she could attain on top of that; and Keltham (though he does not say so) would not want it if she could.  This Keltham has tried casting Eagle's Splendour on himself, from scroll; and he found that +4 Splendour's balance of making his emotions stronger, versus giving him more force of will to endure his own emotions, was more painful than helpful.  He does not want his Splendour increased again.  It makes him more himself again, and he'd rather not be.  There aren't, really, much in the way of feelings that he wants to have.

Keltham's diamond-making takes longer.  He does not have kingdoms and empires offering to be his purchase-agents.

Keltham tells Carissa nothing of that diamond-making, before or after.

He goes to the City of Brass alone, and in secret, and augments only himself.  He doesn't augment Strength or Dexterity, except with a belt he can remove, because the improvement might be too visible.  He doesn't augment Splendour.

What returns from Brass is even less the same person.  He didn't want to be himself anymore, and INT 29 / WIS 27 / CHA 14 enables him to make a good start on that Wish.

The entity that returns from Fommok Madinah—which one might as well go on calling Keltham—does then a thing which intact dath ilani would not do:  He shows Carissa Sevar a Wish-scroll (that he purchased in Brass at ludicrous cost), and asks Carissa Sevar for her help with understanding Wishcraft.  Keltham doesn't name any particular Wish; he says he wants Carissa's analysis about what he can Wish for, and what that would take, and how it might go wrong.

She does not know, when he asks for her analysis, that he is INT 29 and not merely INT 24 like her.

The truth, Keltham has already deduced, is that Wish wordings often seem to be interpreted in a deliberately perverse way, as if some anti-genie were trying to minimize the caster's utility function subject to the constraint of the words spoken.  One can deduce that truth in some detail; there are cases where Wishes did, not harm in general, but something the caster particularly wouldn't have wanted.

You can maybe conjure an exact mass of an exact kind of antimatter in an exact place—if you use Wish-words carefully enough, for the length of the wording is also a conserved resource.

But if it were something more complicated than that, which an antimatter blast alone might not accomplish, you would obviously want to...

And INT 29 Keltham carefully filters which true sources of evidence Carissa Sevar receives to examine, which legends, which accounts.

Carissa Sevar concludes that the caster's conscious intentions are what a Wish perverts.

She herself proposes the clever solution that seems obvious to her, the solution that Keltham foresaw that Carissa would deduce from filtered evidence:  You might be able to get more done, if you'd done everything else right, if at the last stage somebody casts the Wish without knowing what the wording means.

Keltham gives Carissa her last instructions as she stands within a concealing darkness they have Teleported into, to read a Wish scroll whose Wish-phrasing is written in a tongue she knows not.

And by then she does have qualms, even though it was her own plan that brought herself there, but—

Trusting to Keltham's honesty, at the very last, she speaks the Wish-wording he gave her.

And the outcome that comes about is the one that Keltham needed, if not wanted: an outcome pessimal by that Wish-caster's own values.  For the truth was that Carissa stood unwitting far beneath the surface of Golarion, close to where the strange planar boundary of Rovagug's Dead Vault infringed upon the Material.

...And then, with the gods thoroughly distracted, Keltham destroys Absalom wholly in a single blast of antimatter that leaves plenty of safety margin about overwhelming Aroden's protections; and with a third Wish sends Achaekek to Its death; and touches the Starstone with his last mortal thought being: to fix this world or destroy it, bring Pharasma to heel, and tear Asmodeus out of reality at any cost.

The subsequent fight against Rovagug destroys Golarion, of course; but in time It is driven out of Creation before It has consumed more than a handful of other planets.

Asmodeus is tired by that battle, but so are the other greater gods also tired...

...when the ascended remains of Keltham come before Them to present His demands.

Then there is a godwar after a godwar, with Zon-Kuthon and Urgathoa and Lamashtu and most demon lords backing Asmodeus; and the likes of Ahriman and the qlippoth trying to make sure that no god survives it or that Keltham isn't satisfied with the outcome.

Zon-Kuthon fought on neither side of the Rovagug battle, but He fights on this one, and His wrath as a fresh combatant is very great.

When it ends, when it all ends, the first and second layers of Hell are destroyed and everything inside it that was hurting is no longer feeling anything at all.

Axis is ravaged and there are wounds torn out of Heaven; there was a cove in Nirvana where otters frolicked, but it is no more; late-comers to Elysium and the Abyss will arrive to different places than earlier entrants; the deeper layers of the Abyss have been cut loose and cast back into true-chaos; the vast void where Xovaikain once stood is still spreading and unraveling more of the Plane of Shadow.

...But there is no more Asmodeus, and no more Zon-Kuthon, no tortures in what's left of the afterlives; and prophecy across the rest of Creation has been remade to the end of protecting mortals from their own heroes' powers, now that mortals are allowed to rise.

There is little left in that altered Creation for god-Keltham, though He survived the god-war.  Among Zon-Kuthon's last acts was to take up the soul of Asmodia and hunt down the soul of Peranza in deeper Hell, and seize the souls of Ione Sala and Yaisa Castilla fresh-come to Dis; and though Zon-Kuthon had little time or energy to spare for elaborations, He hurt those souls and broke them, and hurled their remains into the grasp of the sort of Outer God that would not just immediately destroy them.

And Zon-Kuthon also hunts the soul of Carissa Sevar from the River of Souls: and working quickly He twists all her desires against herself, makes her something that desires to extinguish itself and not be revived, that longs for a cessation that is not granted; and Zon-Kuthon makes her to be not herself, erases memories or inverts them; and when the consciousness that was Carissa Sevar is ruined as fully as Zon-Kuthon can ruin it quickly, He erases it then, not to continue in any other continuum that respects the current wishes of a sapience.  It was a hard decision, for Zon-Kuthon, whether to inflict on Carissa the fate that Keltham wished least, or that Carissa wished least; but he settled on mainly the latter, for Dou-Bral would have prioritized Carissa's own best wishes for herself.

So there is little left, then, for what little is left of Keltham.  His god-self builds a simpler kinder Lawful Good god that will do whatever Good He might have done, in His place, and then He bids It consume Himself.

Where Keltham's consciousness goes after that—is not something that the little swimming Outer Gods of the Magical Continuum know, where the gods go when they die.

Iomedae would not call it victory; she determined before she touched the Starstone to pursue a better victory than that.  The mortal-ascended gods of Golarion are all dead now, of course, so Iomedae is not there to see it, but She would not call it victory.

Sarenrae, Shelyn, Desna, do not call it victory; Erastil does not call it victory.  Trillions of souls have been unmade within Creation; it's too steep a price, by Their reckoning of costs and benefits.  In Creation's diminished Future there will be mortals who are wealthier and happier; but there were some wealthy and happy mortals already, and more wealthy and happy mortals on the way, if that's a term of your utility function.

Irori is dead, his nephew Gruhastha is dead, almost all of Their students are dead.  It's not too high a price to pay to gain for mortals the right to rise, if it's the only way to obtain that outcome; but Irori would trust to His own diligence to obtain that outcome in due time.  He believes in Himself as strongly as Iomedae believes in Herself (though, Irori would claim, with better justification); and had you offered Him this end, He would have said, "I think I can do better."

Abadar wishes, in the end, in the ruins of Aktun and a First Vault now shattered, that He had never traded with Keltham.

There was a Keltham who would have cared about that, but he ended when he scryed Asmodia in Xovaikain.

Relatively few gods out of Golarion, if They could have seen it all coming, if They had only the sole choice between all of that happening, and not happening, in the beginning, would have chosen not to strike down Keltham upon the spot.

A vast terrible order is overthrown, and a better order has replaced it.  It would be Milani's nature to pay the cost.

Some beings just aren't that horrified by the concept of nonexistence, even before taking into account more exotic possibilities.  The gods can no longer mess freely with mortals.  In time, very few children will go hungry.  The screaming Boneyard babies will have caretakers who are not devils.  Cayden Cailean would have taken it, if offered the packaged choice.  He did not trust to Iomedae's promise of victory with Aroden gone.