Some past posts of mine talked about connections between the ToEE and the Lost Caverns of Tsojcanth, the novel by Thomas M. Reid, a first pass at possible connections between the Elder Elemental God of the Vault of the Drow and the Temple of Elemental Evil, and speculations on the nature of the Elder Elemental God itself. That last will become highly relevant, as you'll see.
I've rounded up some relevant quotes from Gary Gygax concerning his original intentions for the Temple, with an eye towards perhaps "fixing" it. I've cleaned up a few of the quotes to make them more understandable and focused on the ToEE itself, and included some more specific references in brackets, but there are links to the originals if you want to read them in situ.
Thinking back, I believe that the "nine" [in the poem in ToEE p. 66] was meant to refer to Thrommel being the ninth of his line in succession for the crown--and his level, which of course could not be "known" per se. the trouble is, one forgets to set forth all the information created as backstory.
As for Lareth, yes, I did plan to develop Lolth in a different direction. The Q1 module sort of shot that idea down in flames...
She would have made a comeback via the EEG, though, as he was planned as the central baddie in another adventure module...that I never got around to writing (EnWorld Q&A part II)
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I had hoped to get to the Elder Elemental god in a sequel to the ToEE, but... (EnWorld Q&A part II)
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Q: In your original conception of the Temple of Elemental Evil, was Zuggtmoy the big baddie, or did you come up with her as a replacement for Lolth after Q1 was released and you were forced to rethink her involvement?
A: When Dave Sutherland did the Q1 as it was, and Brian okayed it, I was rather stuck. Lolth was supposed to be in there, and in the depths the prison of the Elder Elemental God. I had my hands full with the management of the D&D Entertainment Corp. out on the West Coast, so I couldn't get to the completion of the ToEE. That's when Frank Mentzer took a hand and filled in the lower levels that I hadn't detailed. That's why they ended where they did instead of proceeding downwards more to where the EEG's area was going to be. (EnWorld Q&A part III)
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Q: At the end of D3, the party can end up with the "egg". "In the egg are an iron pyramid, a silver sphere, a bronze star of eight points, and a cube of pale blue crystal." [Great Fane of Lolth, Dungeon Level, Room 5.] The pyramid, sphere, eight-pointed star, and cube evolved into the triangle, circle, eight-pointed star, and square from the ToEE correct [see p.108 of ToEE]? Did you intend the items in the egg to be associated with the elements as they turned out being in the ToEE?
A: When I wrote an adventure I always tried to put in a few disguised hooks for later exploitation, or not, as the creative muse moved me.
As you note, the shapes were repeated in the ToEE as I did intend to tie the latter into the series. Lolth was to be connected to the temple, she the key to activation of that which would remove the imprisoning bonds from the Elder Elemental God. Of course that would have been by unintended consequences of her actions when the PCs discovered her.
How it was all to operate was something I never did get fleshed out. This was to happen in the lower levels of the temple, the development of which I never got around to because of my work out on the West Coast. (EnWorld Q&A part III)
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A: You have sussed out a dark secret! The EEG was indeed meant by me to have a place in the very nethermost recesses of the ToEE. An anomaly there allowed him to manifest a portion of himself, and by doing the wrong (right from the DM’s point of view) thing the adventurers could release him also! Of course that would counter somewhat the freeing of Zuggtmoy, had she been loosed, so on balance it could serve to redress that error. But, alas, I was too busy with other things at the time when the project was being completed. As it was already quite hefty, I decided to omit any mention of this to Frank Mentzer, and so the ToEE was released with only the Eye of Fire as a clue to what I should have included in the adventure. (Oerth Journal 12)So, to boil all this down, it seems there is definitely a connection between the Elder Elemental God that is worshipped by Eclavdra and the drow house Eilservs, and the Temple of Elemental Evil. They share an unholy symbol (the trisected triangle, aka the Eye of Fire).
The Elder Elemental God is partially manifested in the deepest levels of the dungeons beneath the Temple. The magical devices in Lolth's "egg" correspond to the elemental symbols in the Temple, and are presumably the keys to releasing the Elder Elemental God (or keeping him captive).
There are actually two trapped powerful beings beneath the Temple. Zuggtmoy, who was imprisoned by the forces of Good when the Temple fell, and the Elder Elemental God, who took advantage of an "anomaly" below Zuggtmoy's prison and sought to escape his own extra-dimensional prison through here. In so doing, he became stuck. It is possible to free him, however, as well as Zuggtmoy.
I think it can all be put together like this...
In the distant past, the Elder Elemental God was imprisoned on a distant star by the demonness Lolth. The contents of the egg are the keys to his prison. It is only in certain anomalous locations, such as the dungeons under the Hill Giant fortress, the Fire Giant halls, and the Vault of the Drow itself that he can even partially manifest himself, with the aid of magic such as the tentacle rods. Another such anomaly is beneath the Temple.
Zuggtmoy, seeking a suitable "cover" cult, settled on that of Elemental Evil, believing that the Elder Elemental God was in no position to object. She was unaware of his ability to partially manifest. With Iuz as partner, she set her new Temple of Elemental Evil atop the ruins of an ancient shrine to the Elder Elemental God, drawing on a bastardized version of its iconography, not realizing that her efforts were strengthening the imprisoned god, eventually allowing him to partially manifest beneath the Temple, although still unable to actually escape his prison.
After the triumph of Good, the Temple is riven by factions. The four elemental cults are made up of those "useful idiots" who were unaware that "Elemental Evil" was just a cover for Zuggtmoy and Iuz's machinations. Factions loyal to both of those demonic entities are to be found, as well as a faction loyal to Lolth, who sent agents to infiltrate the Temple, thinking it was a genuine manifestation of the Elder Elemental God.
Once he himself was freed from the dungeons beneath Castle Greyhawk, Iuz began his attempts to liberate his ally and restore the Temple, needing a force to keep Furyondy/Veluna off guard with a threat to their rear. Humanoids and brigands begin to trickle back to the place, and spies are placed in Nulb and Hommlet (and beyond). Prince Thrommel is kidnapped and kept in the Temple dungeons for safekeeping.
At this point, rumors of the revival of the Temple begin to spread as the raids by bandits and humanoids under its banners begin anew. The adventurers arrive at the tiny village of Hommlet to investigate...
This gives a lot of layers to the onion. The outermost layer is the Cult of Elemental Evil, which is just a ploy invented by Zuggtmoy. Within that layer are the cults of Zuggtmoy and Iuz, acting as pupper-masters from the shadows. But there are also agents of Lolth, there to infiltrate and undermine what they believe is a legitimate cult of the Elder Elemental God. And in the innermost recesses is the actual Elder Elemental God, trying to escape the prison that Lolth has placed him in, partially manifested in the deepest levels of the dungeons. The surviving factions of the Cult, although currently directionless and feuding, could be brought back in line either by a freed Zuggtmoy, an engaged Iuz, or by stumbling on the actual Elder Elemental God, whom they thought they had been worshiping all along!
In a "fixed" version of the adventure, I would keep T1 (Hommlet and the Moat House) and T2 (Nulb) intact. Some of the material on the upper levels is good, but I'd make the factionalized nature of the inhabitants a lot more obvious, and rework the elemental nodes thing, which never really worked for me. I do love Zuggtmoy's prison. I'd also add a deeper section that was the actual locale of the Elder Elemental God. The players could free Zuggtmoy or the EEG, Lolth could be summoned (and if slain, she leaves behind her platinum egg that contains the keys to the Elder Elemental God's prison...).
And, if the party ever adventures in the Hill Giant Steading or the Hall of the Fire Giant King, they might find those temples beneath them look awfully familiar...