Still doing the thing where we crowdsource some D&D stuff...
Like 'Diamonds Are Forever' Only Not
Shriekglass is a translucent lurid greenish substance made from the solidified cries of dead virgins. It is being smuggled into the city/country/etc. by a ring of thieves (most of whom think it's merely a strange form of amber or emerald).
Do the PCs steal the shriekglass away to interrupt this grisly trade or merely to sell it for themselves? It's not important.
What is important is that the stones are an essential ingredient for the Marilith demon who secretly runs the ring to unleash some form of ritual awfulness on the unsuspecting gameworld.
What's also important is, in addition to the Marilith and her dupes, several other parties are also interested in the shriekglass, for reasons of their own.
All will use guile and deception to approach the PCs and determine whether they have the glass.
The various interested parties and their means of approach are...