The Demonomicon was introduced to AD&D as part of module S4, The Lost Caverns of Tsojcanth. In the published adventure, rumors reach the civilized lands about the ruined lair of the notorious witch Iggwilv, and the player characters head out on a massive wilderness quest trying to find the hidden mountain cave.
I'm going to up the ante for Gothic Greyhawk. The forces of Chaos that have stirred in the land have heard the rumors of the Demonomicon's location and dispatched an otherworldly task force to retrieve the book. Powerful factions across the world take note and mobilize their own teams. Angels whisper warnings into the dreams of the Patriarchs of Veluna and The Pale; the Court of Faeries dispatches a powerful Sidhe Lord to the mortal realm to lead the Wild Hunt; somewhere on the 333rd lair of the Abyss, the Demon Prince Orcus shifts on his throne of bloodstone and smiles. Soon mortal wizards would be able, once more, to open a direct way to his Realm of Undeath, and mortal wizards are so very easy to corrupt.
Making the adventure a race between all these supernatural factions should be a hoot.
I'll be working out who are these nefarious factions over the next few weeks in the blog space, as part of my game prep - I'll definitely reach out to the readers for some ideas as well. There should be plenty of intrigue. Thus far in the campaign, the group has made allies with one of the Fey (opening a fey crossing for a Sidhe noble named the Lady of Dawn); Mordecai had a liaison with a devil-worshiping witch before atoning and rededicating himself to the Eternal Spirit. However, he doesn't have any strong formal ties to the Church yet, and none of the magic users have any strong ties to the known magical colleges. The players should have some interesting choices whether they ally with any of these groups.
I'm assuming that when the players hear about the widespread hunt for the book, they'll throw their own hat in the ring and join the chase. One of the campaign's stated goals was to get the opportunity to engage in the legendary high level adventures of classic Greyhawk. What self-respecting player wouldn't want to claim a legendary tome like the Demonomicon?
The area for this part of the campaign will be set up as a massive hex crawl covering a large swath of the Barrier Peaks and Crystalmist Mountains in Greyhawk (I'll be relocating the Lost Caverns a bit). Such an expedition will present planning and logistical challenges, along with the threat of competition. It'll be like Indiana Jones and the World of Darkness.