Lastwall has never been a wealthy nation, but in the century leading up to the planned return of Aroden, they had some slack to work with.

They mostly planned to serve as a trusted part of the armies Aroden might need in occupying places like Nidal.  But they did at least try to consider some remote contingencies.

At the same time, Lastwall's access to high level wizards has fluctuated over the centuries.  A half century before Aroden's planned return, one not particularly notable eighth circle wizards near the end of his life (insomuch as not particularly notable can ever apply to an eighth circle wizards) wanted to make his mark and leave a meaningful legacy to Lastwall.  He hadn't been very ambitious (again, insofar as that ever applies to eighth circle wizards), and he knew he wouldn't hit ninth circle, he had been comfortably eighth circle for half a century now without any improvements in spellcasting strength.  He doesn't have three impossible tricks, but he has made steady progress for several decades on the subject of anti-Lich countermeasures.

There is one spell in particular he has developed a subtly different version of.  It is well known that speaking a creature's name of birth or using some element to uniquely identify them can boost the efficacy of certain spells and rituals.  Scrying is the most well known example.  Trap the soul is somewhat more obscure, but among those that have the spell it is known that using the creature's name and tricking them into willingly taking a trigger object can make the spell much harder to resist.  His modified version should in principle make it entirely impossible to resist including through both Will and Spell Resistance.

There is one Lich in particular Lastwall has a lot of resources committed to keeping contained.  The Whispering Tyrant, Tar-Baphon.  The spell variation needs the name of the target, and the Whispering Tyrant has wisely hidden his (Tar-Baphon is not the name he was born with).  And it probably isn't worth the effort or risk to get him to take a trigger object.  But a commune with Iomedae confirms it actually is worth the effort of developing this idea as a contingency plan.  There isn't any single purpose spell for recovering a creature's name... but Lastwall has one very general purpose information way of gathering information.  With enough skill in linguistics, names can be broken down into sequences of syllables for each syllable all possible syllables sorted through with yes and no questions.  So 10 communes and over a hundred yes-or-no with Aroden later they have the Whispering Tyrant's name of birth.  (The cost in incense alone is enough to equip a seasoned adventurer, and even having a priest of Aroden run the commune is pricey in divine resources, but Iomedae says it is worth it, and Lastwall obeys.  They at least know communes are much cheaper than a direct revelation from a God, especially of something so secret and precious.)

So, for a century and a half, a magnificent gem bearing the name of the Whispering Tyrant and the final word of the verbal component of Trap the Soul sits in one of Lastwall's vaults.

A century or so after the death of Aroden, The Whispering Way is getting aggressive with attempts to communicate with Tar-Baphon.  They authorize a commune to investigate the possibility of contingency plans, and, after some confusing results, realize embarrassingly they had one lying around already.

It normally wouldn't be worth the risk, but with the Whispering Way getting so aggressive they put the plan into motion. 

They aren't absolutely sure the spell variant really would bypass all spell resistance and Tar-Baphon's spell resistance was said to be so powerful even Arazni could only occasionally land a spell on him.  

The original caster had used some tricks known in his time... the standard (if often obscure) mainstays for boosting caster level: a high quality ley line, borrowed Prayer Beads, and some rodent sacrifices (being able to precast the spell well ahead of time makes some of these much easier to obtain); along with some more recent inventions: a loaned greater rod of piercing spell (a recent discovery then) and a first circle divination for improving spell penetration.  (He too wasn't completely sure his spell variant would quite do the trick against Tar-Baphon mythical spell resistance and used everything he could to make it more potent.)

So they look for methods of improving on it further...

Lastwall has allies.  She is actually a little irritated they didn't reveal this spell variant to her sooner, but will contribute her spells at cost for knowledge of this spell variant (Cansellarion would probably want some uses of this spell off her as well).  She can cast the spell with even more potency with a few more tricks (she doesn't know that one herself, but knows someone and knows a way of sharing the skill), but she is actually pretty sure this variant truly should bypass all spell resistance.  She gets a loaned rod of persistent spell instead of piercing spell (she is significantly less confident the spell variant can entirely bypass Will than that it can bypass spell resistance).  Oh and she has a symbol spell to add.  If they had given her more time she could have optimized it further... in principle the Gem which draws in the soul and body doesn't need to be the same thing as the trigger object... although that might run into the wards sealing Tar-Baphon within Gallowspire... she'll go with what she can do as is, the report about the Whispering Way seem urgent.

Hiding the spells might be tricky.  If they know what the parcel smuggled in is supposed to contain the auras might be confusable... no?  Well, at least she can layer three separate ways of hiding the magical auras.  If she layers them just right it will take 4 separate will saves (thanks to a Rod of persistent spell) and beating her (boosted, obviously) caster level to see through any of them.   But those methods will just be the backup for a rodent (polymorphed down to a size you need a lens to see clearly) she mind blanked and then dominated to "hold" the gem runs into any problems.

Getting the delivery timed with Morgethai's illusion and abjuration durations is tricky, but doable, and worth it.

He realizes in an instant that its a trap, and devotes all of his mythic power into resisting it.  A Moment of Prescience (greatly reduced in efficacy after the breaking of prophecy, but still worth it as a contingency) bolsters his will further and the full might of his mind turns to resisting the spell...  His will turns the spell aside once, but helplessly he realizes the spell his pulling at his Will again, and the Moment of Prescience is already expended.  His last thought is that the spell is far too potent even if his enemies had somehow tracked down his long forgotten name of birth.  His body and soul are sucked away into the gem.

She is watching through the dominated microscopic rodent and is immediately aware once the spell takes effect.  Lastwall had a century and a half ago considered a daring raid to retrieve the gem, but even then there was another potential solution which Morgethai has implemented in advance.  It requires Lastwall to momentarily weaken the seal on Gallowspire, and Cheliax might notice something, but it is still a better option than a risky raid.

The Whispering Way is still a problem in Ustalav, Galfrey has called another crusade with even more haphazard planning than the previous four, and the orc raids are a constant.  But one of their ancient obligations is now permanently taken care of, and their enemies none the wiser to it.

They have a minor incident report to write up about how they ended up forgetting institutional knowledge of their contingency plan, but overall there secrecy has paid off.  Imagine if Tar-Baphon knew to have everything he touched inspected in advance!