I had another thought: Using Death House as a hook.
The House could be in a normal town. Within could be a letters that tell of Barovia (an abbrieviated Tome?) and how to get there, perhaps a black sapphire provides a link, or the ritual is actually a ritual of Summoning the Mists of Ravenloft.
Or maybe a Vistani caravan visits the group afterwards, mentioning mysteries discovered in the Death House.
Or, the characters enter the house in, lets say Waterdeep. And exit in Barovia.
Turns out those cultists where actually trying to summon something evil, but that failed so the evil summoned them...
Which goes great next to the "Strahd is looking for someone strong to follow his footsteps".