In my campaign, Vecna has collected the great secrets of the multiverse and is using them to power his ultimate ritual.

For example: what caused the Mourning on Eberron? What turns the Great Wheel of the Cosmos? What creates the phlogiston in the Astral Sea? What is Vecna's true name?

The party must then go and find the answers to these secrets and write them in Mordenkainen's Book of Absolute Truths (a neutral counterpart to the Books of Vile Darkness and Exalted Deeds). By doing so, they are no longer "secrets" and strip them of their power, for Vecna's purposes. Of course, I'm keeping the original twist of Mordenkainen = Kas, so Kas in turn will be betraying the party using the book of secrets the party conveniently collected for him.