A few entries in the World of Sulerin index have caught my attention. Most of them seem to come from a book called Doors to the Unknown. They refer to creatures who come from something called "Beldaari" or "Hyper-Reality."
Does anybody know what this is, and whether it fits in any way into Planescape cosmology? Is it part of the Far Realm, or is this Doors to the Unknown book from some non-DnD game like Call of Cthulhu?
If you want to investigate the concepts of Hyper-reality read some of Jean Baudrillard's work. He came up with the concept. It's pretty heavy going if you're not used to post modernist/ post structuralist theory so I'd probably just use Wiki or something, that should give you enough grounding to use it intelligently in your games without having to invest too much time in a fairly esoteric topic (or you could do a sociology major and be able to talk about this crap for hours while no-one else actually cares what you're saying).
[edit] interesting side note- I believe the concept of Hyper-reality was first introduced in the book "Simulcra and simulation". This was the book that Neo kept his disks in at the start of the first Matrix movie, a nice touch given the nature of the Matrix
What's the fifth level of hyperreality?
From what little I know of Jean Baudrillard's work, its connection to the DnD notion of hyperreality seems to be in name only. These five levels of reality do remind me certain aspects of some branches of New Age thought.
D&D seems to have sanitised it and taken away some of the more depressing/ confronting aspects (as you would expect from game manufacturers, it has to be at least somewhat fun to play). It's also been tweaked so that it can fit into a cosmology which promotes adventure; however the key concept- being more real than reality- remains the same.
Feel free to ignore the original theory if you like, it probably won't influence your gaming experience too much although it might add a bit more depth.
To be honest I wasn't going to reply to this thread until Rip stated
They seem to have taken away every aspect of it other than being more real than reality itself; and even then the way the hyperreality of DnD is more real than reality is completely unconnected to how Baudrillard's hyperreality is more real than reality. But I know where you are coming from; I constantly do the same type of thing.
Cool. Thank you. 
This reminds me of the Spire-Builders sect somebody posted on the forum. It would make sense that level-3 magic and psionics would not work on the fifth level.
But what is the fifth level... or is it unknown?
The 5th Level of Reality is largely unknown and unaccessible to mortals, but legend (specifically the Nightmare Lands Ravenloft box set) says that the gods dwell in Level Five hyper-reality.
The impression given is that its so far above the normal level of reality that mortal minds can't cope with experiencing - almost like having Level 5 reality is what being a Power is.
Doors to the Unknown is a Planescape adventure. In it, a being exiled from a hyper-reality realm in Mercuria, the second layer of Mount Celestia, causes trouble in Sigil and elsewhere. The realm is called Beldaari. The race of of hyper-real creatures is called mercurials. Other inhabitants of Beldaari include good incarnates and creatures of raw happiness known as forest wisps. The realm of Beldaari belongs to an unnamed god of pure thought.
The concept of hyper-reality comes from the Ravenloft boxed set The Nightmare Lands, which defined five levels of reality.
Level one reality is common dreams. They are incapable of hurting creatures from our level reality.
Level two reality is dreamscapes, which are dreams that can hurt you. There's more on dreamscapes in A Guide to the Ethereal Plane and Heroes of Horror.
Level three reality is waking reality, "our" level of reality. Of course, our minds spend a great deal of time in levels one and two when we're asleep.
Level four reality is hyper-reality, more real than real. Compared to hyper-reality, our reality is only a dream, albeit a very vivid one. Everything in hyper-reality is more intense, more vivid, more damaging, more colorful, and so on. Hyper-real creatures like mercurials can manipulate our reality at will.
Hyper-realities are rare in this multiverse, generally occupying out-of-the-way corners of existence and not affecting the rest of the planes very much. When they do, the results can be catastrophic.
Beldaari is just one hyper-real realm. There are others.
Hyper-reality has nothing to do with the Far Realm, although I wouldn't be surprised if the Far Realm had hyper-realities in it.