Sharne's Clockwork Market - Intro

Here's an idea for another module in the vein of Ynn and the Library

Sharne is the ghost of a world. Fundamentally dead and barren, with nothing left but to dissolve before the remorseless caress of entropy. Once a functional reality like many other pocket-realms, it has reached the end of its life-span.
Above a dry, dust-choked ruin a red sun hangs bloated and low in a foggy sky. There are no plants, save for the desiccated skeletons of long dead trees. No birdsong, nor scavenging animals, nor even the buzz of carrion flies. No breeze disturbs the streets as, brick by brick, the place crumbles away.
In the distant past, when humanity first carved writing into stone tablets, Sharne was a bustling place. The residents were tall, elegant beings, as if carved from jet and alabaster. Possessed of alien grace and minds brilliant like the blue flame of a blowtorch, and a mastery over magic and science that rendered them the undisputed masters of their domains. On earth, the fledgling races of mankind called them by many names; the Annunaki, the Sidhe, the Shining Ones, the Nephilim.
They built this place - using the same huge machinary as that of Ynn, Stygia and Laputa among many others - to be a thronging bazaar, a densely-packed market-town where they might buy and sell anything, no matter how esoteric. For an age of the earth, it was at the centre of a web of trade that extended across many worlds.
Eventually, the civilisation of these beings fell to decadence, then to cruelty, and then to civil war. As their world-engines ground into an all-consuming conflict, ever greater weapons were developed. Black-holes of the mind that negated all meaning, memetic viruses that would drive all who knew their secret to insanity, quantum-prions that reconfigured atomic matter into thrashing plasma, slaved titan-minds capable of unthinkable feats of will and strategy.
Worst of all, the deplorable weapon. The forbidden spell, Power Word - Kill Everything.Armies gathered in the rolling plains around Sharne, each vying for control of the trade-dimension. One commander, seeing their forces outnumbered and cut off from retreat, in a fit of imperious rage, spoke a single syllable, and the world died. 

How do you get to Sharne?Reaching Sharne is not easy. A prospective visitor must demonstrate that they have the means and willingness to trade. To open a gate, the following steps must be followed:
A gateway, with two pillars and a lintel, must be constructed out of ivory. From the lintel, a curtain of silk must be hung to obscure the other side of the portal. The whole construction r will likely cost around five hundred silver in materials and labour costs.
Once created, the gateway will always function so long as it is not damaged.
To pass through, the entrant merely need speak the words 'I seek entry to Sharne' and step through the curtain.
There is a toll to enter, paid every time for each entrant. The machinery of Sharne claims something intangible. (There will be a table for 'intangible costs', things like memories, years from your lifespan, slivers of your soul, artistic inspiration, and so on. You roll each time you need to pay something like this, and might adjust your stats accordingly). Those unwilling to pay the toll find themselves entangled in a silk curtain in the real world; those who pay pass through and find themselves in the bleak ruins of Sharne, an open door of ivory at their back. 

Why go to Charne?Although dead, the ghost-world is not entirely uninhabited. Those things which never lived were unafected by the deplorable weapon. The trading automata stand vigils, their inner workings slowly rusting away as they wait for customers that may never return. Here, by discovering the right machines, an explorer might purchase all manner of esoteric rewards. Beauty, genius, and murderous potency can all be purchased here. Among the slow-collapsed ruins, the encroaching dust and the piled rust and rubble, all manner of fabulous treasures can be found.
And there is life here, of a sort. Not native life, but rather things that found their way in from other worlds. Sometimes, a merchant-prince of impressive means from the real world will lead a caravan through elephant-sized ivory gates. They come in search of rumoured wealth, driven on by the rumours of their fellows. Some return, others remain in the sand-choked streets forming their own bizarre culture of pauper-merchants. There are stranger visitors, too, drawn to long-fallen Sharne in search of whatever their society prizes most.

That's the basic setup. The obvious inspiration is it's namesake Charn in C.S. Lewis's The Magician's Nephew. Likewise Jack Vance's Dying Earth novels and the paintings of Zdzisław Beksiński. I expect that there will be some references to modern automated commercial culture; vending machines, drive-thru restaurants, ATM machines. The dehumanising effects of unbridled capitalism. Clockwork and throbbing tubes.
And, at its heart, in the deepest recess of the adventure, will be Power Word Kill Everything. Here's what it does:

Spell, level 1. Learned from scrolls or spell-books like any other MU spell. Clerics who encounter it likewise add it to their repertoire. Same for other variant spellcasters and wielders of magic, although you might need to bend it into shape to fit odd magic systems. Upon casting, immediately and without any possible countermeasure, every living thing in the reality inhabited by the caster dies instantly. There is no saving throw allowed. Resistance to magic, counter-spells, contingencies, and extraordinary resistances do not apply. Upon casting, all living things in the caster's world die. (Non living things such as robots and zombies are unaffected. This need not end the campaign, as it only affects the caster's current world, plane, layer or dimension.)

So there's some treasure to give your PCs. Think of it as a mutually-assured-destruction insurance policy against railroading.