In the Lost Caverns of Tsojcanth, the players are sent to explore the lair (or at least, a lair) of the long-dead archmagess Iggwilv, where they find her daughter (either natural, or some suggest adopted) Drelnza, cursed with vampirism and held in stasis. Iggwilv, it turns out, is also the mother of Iuz, the evil demigod who rules the lands east of where Tsojcanth is located. Iuz's father was the demon Graz'zt ... who was later entombed in a stone slab in Castle Greyhawk.
Tsojcanth is also surprisingly close to the Lost Temple of Tharizdun (there's a lot of lost stuff in NW Greyhawk, apparently), and from what I know of that module (I never read it), I would expect that Iggwilv probably knew it was there. That area is also now home to the archmage Mordenkainen, who was very likely pulling the strings behind many various player-character forays.
Meanwhile, in the original Temple of Elemental Evil, part of the backstory is that the Drow goddess Lolth had "recently received a severe blow," which is generally read as a reference to the events in Vault of the Drow. There are also references to Iuz intending to use the ToEE as an ally against Veluna and Furyondy, and Iuz himself makes an appearance near the climax of the adventure. VoD is the followup to Against the Giants/Descent Into the Depths series of adventures, which start in Geoff and go on to parts north and west ... just a stone's throw (in giant terms) from the Yatil mountains (where Tsojcanth caverns are), and just slightly further away from the Empire of Iuz, Veluna, Furyondy, and of course the ToEE.
I just love the way it almost clicks together; the way Tolkien trivia hounds like to examine the many facets of Middle Earth, puzzling out details of the relationships between Numemor, Angmar, Gondor, and Dunland, I like to puzzle out just what did happen in the northwest Flanaess. The fact that it isn't all spelled out just makes it that much more tantalizing. If it was just laid out, it would just be a narrative to be read.
I also like the way that many of these Important Personages and shadow players are not just straight good or evil. Mordenkainen is no Gandalf, advising the good kingdoms of humanity against the evil of Iuz; depending on how it may serve his esoteric purposes, Mordenkainen will back Iuz in one fight, then turn around and back Furyondy in the next. Iggwilv was power-hungry and obsessed with magic ... but she also suffered from a broken heart. Did she love Drelzna, and place her in stasis until her vampirism could be cured? Or did she hate her, and inflict the vampirism upon her? Different sources give you different answers.
Meanwhile, my current campaign is set in Bissel, a little valley nation right in the center of all this mess, the Belgium to Greyhawk's 1940's Europe. It's set 15-25 years after all those scenarios, and features a new generation of would-be heroes. This gives me a lot of history to mine, should I choose to ... but since jamesbarrett is the only other member of the group with a heavy dose of Greyhawk knowledge, it also gives me the flexibility to ignore anything I don't want to bother with.
So ... where to, my would-be heroes? :)