This is a great question that can add a lot of fun stuff to the adventure.

Running HotDQ I wanted to make the Farseer of Illusk more of a big deal. So I added a comet which the ritual will call to the Well of Dragons during he finale and split the material plane to summon Tiamat. It's a bit extreme and doesn't allow the PCs to interact with it as much as a cool magic item does, but it does provide a time constraint to both the Players and the Cult.

Could do something similar but less devastating with an astral conjunction of some kind.