u/Schleimwurm1

The characters become aware of the cult activity because of general rise in crime, everywhere. Robberies, thefts, brutal, elegant, puracy, insurance fraud... whatever. What is scaring the NPCs is, no one does anything with that. No Castles being bought, nor armies being raised etc. This gives almost infinite quest hooks. Some of the mundane goods end up on the Caravan, so there are different ways to follow the thread.

The stolen goods are being used for one purpose only: to smelt into a Black Dragon Mask (takes a Black Dragons Breath, and 10000000 GP). The OG mask was destroyed, and the other masks are around. (Made Severin a Rakshasa that stole the Blue Dragon Mask out of Sorcerous Sundries in Baldurs Gate)

There's a Dragon Turtle in the sea north of Baldur’s Gate, and therefore all trade towards North has been limited. The Caravan is the first Caravan going north in 6 months, transporting the rarest of goods, where the margin of profit for the merchants is just too great not to risk the wilderness. - This makes the caravan a little more of an expedition, rather than "we're mall cops for 2 months".