Plot Notes

-The Iron Sphere had no special properties ("nothing more than crudely-worked sky iron" - it's 2.5ft across)
-Pafthinel was hired by the House of Orjo to enchant the stone so that it could be used as a bit of a "trojan horse" to enable an invasion of Sthombo. The cities were making peace but Hazard wanted to get revenge on Sthombo.
-the Stone acts as one end of a portal - it can be moved to wherever
-the second end of the portal is the Shrine of Culsu in Hazard; the proper arrangement of magnetic stones in the Shrine will open the portal to wherever the Stone is. ("I was able to attune the Sphere to a fixed point in the Werld. A proper arrangement of ordinary magnetic stones at that point would collapse the magnetic field around the Sphere into a vortex, creating a conduit between the two from anywhere in the Multiverse")
-Culsu is "patroness of doors and passages, but her true divine station is to stand at the gateway between the Werld and the planes beyond" - so an obvious link to portals.
-But the portal is bi-directional; it could be used to send whatever is at the Stone's location to the Shrine. I think we should assume that the 6PC has the knowledge to open the portal.
-Destroying the Stone is our best bet - Pafthinel also suggested dropping it in the ocean, but my fear with that would be that the portal can still be opened and let in the ocean/a Kraken, etc.
-Moving to speculation, I think the Cult's plan is to open the portal once the Stone is act a location that would cause both a natural disaster (flooding) and possibly one that involves a large magical beast that will wreak havoc

So the 6PC nicked the Sphere now intending to use it for their own purposes:

Garho looks skyward, thoughtfully. "It comes in the part where she is describing the royal court of Hazard. She has been asked to read their fortunes, and when it comes time for the king to have his fortune told, he tells Aligoon about his brother-in-law, a man learned in the ways of magic, who had been exiled years ago for disloyalty.

"The man was cast out because he had become a zealot, and was known to say that 'only the wearer of the six-pronged crown is fit to rule in Bload.' He settled in Sthombo after his exile, and lived freely there as a sage and healer, styling himself as 'the first servant of the six-pronged crown.' He was well-liked in the city, and had the ears of its lords, and the King of Hazard was worried that he was plotting intrigues and vengeance against him, so the king asked Aligoon to prophesy the fate of the 'first servant.'

"They brought out some old jewelry that had belonged to him, and through these objects Aligoon looked into his future. She told the king that before his reign was over, the First Servant would be entombed alive within the crypts beneath the Temple of Nethuns. The king was satisfied. And that is all that Aligoon had to say on that subject." Garho smiles. "Until many years later, when she writes her marginalia. This is what caught my attention, relevant as it is to the interests of my profession.

"In the margin, she writes that she has since learned the meaning of the 'six-pronged crown.' It refers to a tale told of the eighth circle of Hell. Long ago, during some long-forgotten infernal war, a colony of ice devils were called to muster at the fortress of Caltlikrath. For some unknown reason, six of them strayed from their path and encamped on an isolated glacier. They slew the devils sent to admonish and retrieve them, holding the glacier as their own fiefdom in perpetuity thereafter. Eventually, the glacier came to be known on Hell's maps as Seglogron. Which translates to 'six-pronged crown.'"

Garho reaches into his garments and removes something from around his neck: a wooden pendant of a fish with a hooked snout. He passes it to Leth. "If you wish to find me again, pray to Bahanzin. Perhaps he will send me to you. I have few friends on Bload, but if ever you find yourself in Felboc, seek out Velenix Silver-Chain. I have not met anyone in this land whose expertise in goetic matters matches hers."