When I ran a particular 3.0 mega dungeon (that has already been spoilered earlier in this thread) I ended up elaborating on the Big-T worship aspect of the campaign, even going so far as to chat with Gygax himself (through e-mail. What an awesome guy, to just oblige a random DM like that) about the Dark One.

I've lost pretty much all my notes in the decade since then, but I'll go over some things I remember. I forget what info I got from source and what I came with on my own. Most of it was from one source or another. Just for fun, I'll write this all from the perspective of a mad cultist.

There is the Great Wheel. Infinite in many aspects, but those of us who are enlightened can easily understand the concept of multiple infinities coexisting within the same multiverse. The Abyss itself is said to hold infinite lairs, each of infinite space. And this is but a spoke on the Great Wheel.

But when looked at from a whole, the Great Wheel is but a small pebble in the sea of an Omni-infinity. But what lies in this dark ocean outside of everything that is? It is Nothing. And His name is Tharizdun.

The Chained God, my Master, is Everything that is Not. He is the All-Nothing. And He is far greater than what Is. The Gods we know have carved a multiverse from a small wound in His side, but the one inevitability of creation is that it will one day be unravelled. But fear not, for we should not try to escape the unravelling! Nay, for only when we are unmade will we be returned to the Great Nothing from which we were stolen.

You see, my Master, he is not chained within some limited prison, as many "studied" scholars seem to think. How small minded are they! They think that the Great Wheel is all that is! No, my Lord has been chained without of this fragile shell, locked into the infinite infinities beyond! He is being held away from this existence by greedy hands, who still seek to steal His power, and hide it from Him.

Before creation, the children of the One God rebelled against Him, and against His vision. We do not know what he sought to do with this power - but I can only guess it would have been beyond description! But the jealous child gods stole the power while their Father slept, and made the Great Wheel in his shadow.

Wounded, was the Great God, and he sought to reprimand His children and undo the travesty that they had done. The children couldn't even form a unity between themselves on how to use this power that they had no responisbility for. The mockery they call Pelor - the eldest of my Great Lord's children, created an uneasy alliance between his sibilings, and together they locked the gates to their Piteous Wheel, so their Father could not correct what they had done.

Soon after, these childish gods forgot their Father, and resumed their pathetic squabilling with each other in their tiny toy house.

But, fear not, my dear friends, and followers of the Great Nothing! For every lock has a key. The Theoparts. Holy relics that are but a fraction of the Lord's power that was trapped within this world. When the three of them are reunited, the door to this reality will be revealed, and Tharizdun will finally be able to know the way in, to right what has been wronged. And he will not forget his loyal followers - that aided him even when he was unable to speak to us!

But we see his works, even so. The beautiful gateways that scholars call Spheres of Annihilation - holes in the fabric of the Great Wheel... they are the signs that our Lord is finding his way into the world - that he will be here soon!

Also, some of our more blessed members, have been visited by the Dark One in dreams. He appears as a seven foot tall striking blonde man in black armor. The path to his haven is a solid rainbow of light, and his glorious castle is crafted of pure crystal, resting on a cloud. The only thing he is able to ask about is his key - the theoparts - and he demands that we should bring these relics together!

Another piece of his power was able to form into the being we know as the Elder Elemental Eye - but this powerful being is like a lost child, forgetting it's true place and purpose. Nonetheless, it is our Lord's true avatar within existence, and we should seek to do it homage. Unfortunately, the 333rd layer of the abyss - it's domain - has been sealed off to all beings. I suspect that there is a weak point in the structure of the Great Wheel somewhere on the plane. If only there was some way to gain access!

But do not despair, my friends. We will prevail. There is no stopping it, not even if all the gods were to rise together again could they stop His power. We will be unravelled. And when the Great Tharizdun remakes the world in His Image, according to His Design, we His followers will be given sacred places on high as our eternal reward!

Oh, Great Tharizdun - Entropy to this Accursed Existence - we will usher your way soon, so you may Unravel this unworthy existence that was stolen from your Holy side!

I think what makes these theories most compelling is there is truth to them. Big T is basically an Overdeity that wasn't invited to the party, so he wants to wreck everything and do it his own way. Whether or not he'll "remember" the followers that helped him is another matter entirely, though.

All in all, Tharizdun is a great concept. It worked especially well in my campaign when the PCs started to see some merit in the crazy cultist viewpoint. Anyway, I hope this mad cultist's ramblings were enjoyable at least, if not insightful.

Sorry that ended up being pretty long...