If I ever find more writing time, this is how a sequel novella to "Castle Greyhawk" would go:
Ch. 1. Gronan and Ayelerach meet in the mostly-cleared out Temple of the Elder Gods, besting the type I demon that slew their henchmen. They comment on how little they’ve heard from adventurers in Blackmoor and Kalibruhn in the last year, bemoan how weird the monsters are getting in the dungeons, and figure there are 16 free adventurers, not counting henchmen, wandering the Greyhawk area. They debate having their henchmen raised from the dead, but don’t know who to with Crom no longer answering his clerics.
Ch. 2. Terik comes back to Greyhawk from the other side of the world, finds Tenser at his castle, and tells him of his experience meeting Zagyg and Zagyg’s warning that the world is changing.
Ch. 3. Robilar is stalking prehistoric mammals on the Wild Coast, but finding them rarer and rarer when he stumbles onto a bronze age village that should not be there. He even witnesses some cavemen seemingly evolve before his eyes.
Ch. 4. Terik brings Tenser to the Temple of the Elder Gods, but can’t open the portal to Formalhaut.
Ch. 5. Byrne and Rufus adventure in Hommlet.
Ch. 6. Robilar flies back from the Wild Coast in impossibly short time and joins in at the Temple of Elemental Evil. He is perplexed by this "Cuthburt" god the locals keep talking about.
Ch. 7. Robilar, Terik, and Tenser return to Castle Greyhawk and find the dungeon entirely different. They explore a new 13th level (with fountains of fire?).
Ch. 8. Robilar and Tenser seek answers from Mordenkainen in the Barrier Peaks. Mordenkainen knows little more than they do, but that seven adventurers who came directly from Earth are on Oerth and may know more. He has divined they are in the newly discovered land of Perrenland.
Ch. 9. Robilar, Tenser, and Mordenkainen find Milo Jagon, Naile Fangtooth, the elven ranger Ingrge, the bard Wymarc, the cleric Deav Dyne, the lizardman Gulth, and the Amazon battlemaid Yevele only to be attacked by ogre magi working for Yggwilv. Yggwilv steals the dice bracelet from Gulth.
Ch. 10. The adventurers all track down Iggwilv, who uses the dice bracelet to summon the border god, Saagrash (who Robilar and Mordenkainen know), to fight the heroes. But Zagyg arrives to announce that the world has finished a shift to a new age and Saagrash is transformed into Graa’zt and takes Iggwilv with him. Zagyg invites everyone to see Mordenkainen’s map of the world and how it has changed.
Thanos by Al Milgrom
19 hours ago