Exactly, they are for the most part unknown. For anyone wanting to play a 30+ campaign my suggestion is that you lift the veil on these mysterious beings.
My suggestion is that there are certain beings that are so powerful, so awesome that the are always, and will always, be beyond the reach of even the gods themselves.
Would level 31+ PCs be 'mortal'? I don't think so.
You don't become a god just by getting more levels. Gods aren't just beings with more levels than the PC's, they are an entire other level of being that isn't just adding 1 to your level count. Will you naturally die of old age if you don't use necromantic trickery to cheat death? Will you die if I plunge a sword into your heart or take your head with an executioners axe? Will you die if a God of Death says your time has come and really means it? If the answer to any of these questions is "yes" then you are mortal.
Obviously something has to be able to affect these beings, otherwise their inclusion (as opponents) is pointless.
Most of these were never included to be direct opponents, most of these beings have always been narrative devices and not antagonists. Lord Ao, Overgod of Realmspace, is an uber-plot-device used to explain edition-level changes. By fiat alone he was able to strip every deity in Realmspace of their divine rank and exile them to the Material Plane, and the closest he got to stats was in the 2e Faiths & Avatars book that essentially said his stats are "He wins against any mortal or god, end of story". The Lady of Pain was explicitly and intentionally never given stats because she is so beyond any conceivable ken that there was no point in it (it wasn't until 3e she was even given an alignment of LN). The Dark Powers of Ravenloft can never even really be "confronted", the only way you really "oppose" them is to try to escape from Ravenloft (in some Ravenloft campaign styles), and even then are you winning or are they letting you out?.
None of the above remotely begin to tax my divine hierarchy.
Mortals < Demigods < Gods < Sidereals (aka Overgods) < Eternals (aka Time Lords) < Supernals (aka Supreme Beings) < Ultrals (hypothetical strata).
So you have your own homebrew divine hierarchy that supports your ideas. Have lots of fun with that in your home campaign. For purposes of the normal D&D worlds and multiverse however, that really isn't relevant.
As already mentioned, that is for the imprisoned form of Tharizdun. Try reading the Gary Gygax' Gord the Rogue novels (specifically Dance of Demons) for more information on Tharizdun.
I am sure in the Gord novels Tharizdun is made out to be a very, very powerful deity because it is a necessity to establish an antagonist as powerful as a literary device, but powerful on the scale of an Overdeity that creates entire worlds, could destroy the world and start over if it really wanted to, can elevate and strips pantheons of power, and can make edition-level changes to reality?
Thats because you are thinking of levels in a linear fashion. Think of them as a pyramid tapering to a point...or at least to a plateau with a big eye at the top.
In D&D, levels are linear. I have actually played 100th level AD&D characters, in 1e and 2e (Throne of Bloodstone). Those characters have more HP and NWP slots than 20th level characters, but not much else. If you use the High Level Campaigns rules from 2e then they are a little beefier by 30 than they were at 20, but from 30 to 100 it doesn't spiral up in power that much. In 3e, a 100th level PC has more BAB/Epic Attack Bonus, HP, Saves, Skill points ect, but it's linear progression. In AD&D, 100th level PCs are still very vulnerable to dying from divine-level threats (which Throne of Bloodstone throws at you), and even a 100th level party might be able to take on some of the deities in 3e Deities and Demigods or Faiths and Pantheons, but some of the more powerful and combat oriented ones with Salient Divine Abilities that let them kill any number of nondivine beings across any range (including interplanar) with no save or SR, or can regenerate from any injury or even apparent destruction inflicted by any being that isn't a deity of their own rank or higher, those kind of abilities mean that there are some deities that mortals (i.e. anything without a Divine Rank) really just can't fight even if they were 1000th level.
If overgods are to gods as gods are to mortals then it stands to reason that a group of powerful gods could do battle with a (relatively weak) overgod. Just like a group of mortals could battle with Demogorgon, Tiamat or Vecna.
The problem is, it's been explicitly stated many times over in the source material that these beings are so powerful that they can defeat even the mightest god with negligible effort. The Lady of Pain can, and has, killed Greater Gods in the blink of an eye. Lord Ao merely made a proclamation and the countless deities of Abeir-Toril were instantly stripped of Divinity and exiled to the Material plane until he decided otherwise. They exist on a level of reality so removed and transcendental it would be akin to even the mightiest boss monster in World of Warcraft attacking you the actual player in reality from your computer. No matter how mighty it is to the characters, to a person in our reality it's just something we can walk away from or turn off and ignore even it's greatest attacks as a pretty picture.
The only real examples I can think of a being on this scale being directly opposed are of Chaos in Dragonlance and The Snarl in Order of the Stick (which may well qualify as a being of this class due to it being formed from the collective power of all the 4 pantheons original deities of the world, and I was trying to keep this to official D&D worlds, but the Snarl does make a very good example of this idea.) Both of those are ones that are particularly mentally weak and fooled/deceived/manipulated. The Snarl can kill even Greater Gods with one swipe of it's claw (it killed Zeus in one hit IIRC), and if unleashed could unmake the world in only a few minutes, but was able to be imprisoned first by three pantheons working together in concert, then when that prison began to fail by an Epic levels party and exist as the Macguffin in the main plot of the series. If, and that's a big if, you want to have one as an antagonist you would need to give it huge, gaping flaws that make it weaker in one or more aspects than most beings of it's class such as giving it a childlike or animallike intellect to go with nigh-infinite power instead of the limitless mental prowess most beings of this class have.
There are any number of interesting ways to bridge these gaps.
If you want to make D&D a game of ever increasing levels taking on yet more and more powerful deities and overdeities, sure, it's your game and have fun. However, with the established and conventional D&D settings and multiverse there are some places mortals shall never go, and some things that really are always too big.
