Now:

When last treaty was made between gods, Iomedae swore not to open Zon-Kuthon's vault, nor to connive toward freeing Zon-Kuthon.

The agreement doesn't, technically, say that Iomedae can't do this, which is not something that even gods would have foreseen Her doing.  This scenario required several sequential then-unseen implausibilities, and if you negotiate about every combination-possibility like that, you really will end up negotiating too much.  Gods are large, but not large enough to defeat combinatorial explosions.

The gods would have expected, if Iomedae gathered Herself and went inside the vault, that Zon-Kuthon would promptly eat Her; for She is smaller and younger and would be to Him as a snack.

But what actually happens -

Long ago, at least as mortals count time, a young human girl knew (it was not really a matter of deciding) that the right thing to do was spend all her life trying to fix everything everywhere.  And some time after that, she knew the right thing to do was become a god and go on trying to fix everything everywhere; though she didn't think that, even as a god, she would live to see it fixed...

A furious blaze of light, shining brighter than it did in Heaven, enters into Zon-Kuthon's vault where a great Evil lies wounded and helpless.

Some other possible god of paladins might've found this part difficult, contrary to Their domain; but Iomedae never was the god of fighting Evil.

She falls on Him like a mouse nibbling into a comatose elephant.

OKAY WHAT NO HE SUDDENLY HAS LARGER PROBLEMS, NAMELY IOMEDAE IS CONSUMING THE WHOLE POWER OF AN ANCIENT GOD—

She doesn't see how Iomedae can get away with this, but has little choice but to back Her once it's begun—

Even if Iomedae somehow survives this, it will trigger new divine treaties about Golarion's broken prophecy and divine opportunism, which won't be to Chaos's advantage.  But if this is Cayden Cailean's last wish, though She doesn't understand it, then to defend Iomedae She will also go—

Lawful Evil, Lawful Good, what care I?  You may go about your conflict there, while I take care of this Lawful Evil nascent here.

Still no.

You think you can stop Me, jumped-up mortal?

I know that I can stop You for long enough.

The lights of godwar flicker on in the sky over Absalom, focused on Absalom, turning day into a greater and more terrible day; even as fire spreads outward from the ruins of the Ascendant Court.

And Pilar Pineda lays upon the Starstone and—

She is—

She is in many places—

She dreamed of this, Snack Service showed her—

Showed her—her own thoughts, somehow, as if all of this had happened to her before, in some forgotten life that Snack Service somehow remembered—

She had known this—she had prepared for this—she is Pilar Pineda Pilar Pineda Pilar Pineda was once a single person and is now some strange fractally shattered mirror-of-herself, expanded and also broken, must pull herself pull herself together—is not large enough, to pull herself together—

She needs more power—

(The draining of divinity is a motion that Pilar Pineda has practiced some little bit—)

The Starstone glows, the buried awful core at its center pulls, and Achaekek's death pours toward it and flows also into Pilar Pineda where Her mortal shell still lays, making of Her something that is greater than Cayden Cailean or Norgorber became, greater than Iomedae as She previously was.

- He may have been wrong about which of His problems is bigger.

Asmodeus is not slow to notice when gods are moving against Him. The timing of this is meant to divide His attention or limit Him in responding, to one or both exigencies.

If He doesn't intervene on the baby godlet (and He's still not sure whether He'd gain from crushing it or preserving it), He perhaps loses; if He doesn't intervene to stop Iomedae, He surely loses; and perhaps both events are a distraction. But identifying that as the situation doesn't save Him from making the call about which to address.

...He can at least fire off some large offers of payment to various Lawful entities aimed at 'make other things stop happening', to limit the damage His distraction can do to His interests while He crushes Iomedae, which really needed doing anyway.

And Cayden Cailean dies, manifesting at the very last in the form of His mortal self as He was when He touched the Starstone, standing over the Starstone in that same place; holding His rapier crosswise in hand, as though to protect Pilar Pineda.

And then He crumbles away.

The power that was in Him begins to be sucked back to the Starstone; but the vast flickering shadow of the Mother of Monsters descends to consume that, consume His corpse, feast upon the dead as was always Lamashtu's way—

I think not.

It is the voice of something more powerful than Cayden Cailean ever was; a goddess who bears about Herself the portfolio of Lawful Evil, a portfolio within which Achaekek's and Zon-Kuthon's deaths have made great room.  And Acavna's domain of Companionship, and a trace of Acavna's defensive battles now transformed to a yielding endurance; and Cayden Cailean's last gift to Her, that is His domain of revelry, and of lust; and from dead Zon-Kuthon the domain of sadomasochism, but a different kind than that practiced in Nidal.

But she is goddess first and foremost, not at all of enslavement, but only of being a slave.