All my party knows is that the Cult is trying to bring Tiamat back to the Material Plane. They have no idea how, why, or where.
I don’t think the published campaign has any details about how the summoning ceremony happens, other than it involves the Dragon Masks. I’ve already changed that up a bit as my party managed to successfully retrieve two masks so far (white and black). In my world, Severin doesn’t need the masks to bring her over, they only help determine how powerful she is.
I’m not sure if stopping the summoning is something I want the party to be able to do. That sort of eliminates the big finale! They definitely can impact Tiamat’s strength when she arrives and how many allied forces they have, however.
Alos, I don’t think there’s anything in the published campaign about a role for the Devils or Demons in the Cult‘s plans. I suspect the Demons may want her to stay in Avernus because the Devils have to expend power to keep her there.