Pratchett has this notion of “the place where the cthulhoids come from”, the Dungeon Dimensions. It’s the layer below reality, where things with tentacles for faces rule. D&D has taken to calling it “The Plane of Madness” or “The Far Realm”. The thing that’s funny about D&D’s take on it is that it elevates aberrations above demons and even elementals as a more fundamental order of being. You have your little orrery of a world, the heavens, the hells; and then behind and around it all, an endless sea fundamental madness of everything, the far realm.
Frankly, it’s annoying. It lets Cthulhu win. HP Lovecraft’s point was that on the grand scheme of things, the universe doesn’t care about you — it’s ambivalent at best and inimical at worst to life in general and humanity in specific. What you do doesn’t matter, you aren’t the apex intelligect; you are bugs. That’s fine, but that’s a specific stance on the place of humanity in the cosmos. When the game has The Gods themselves as entities of the same order as the PCs (consider: humaniform, capable of spellcasting, not generally super-psionic, living on a fragment of the astral plane), introducing the gribblies at the border of reality is taking a strong stance.
Not a bad stance! Just a strong one.
My take is a little different. I like people. I think that D&D doesn’t work as well in a universe that has made up its mind that sentience doesn’t matter, that players don’t matter.
My astral plane is pretty close to my ethereal plane and my shadow plane, one big blob called the Nether. In some places the nether rots: that’s the plane of shadow, and undead come from there. In others, the nether elevates and clarifies: that’s the astral plane. In some places where it clarifies, however, it clarifies wrong, balooning out. Those pockets of madness are themselves aberrations, and creatures that come in contact with them are also aberrations.
The nether — the astral — plane is the minds of creatures. Where it goes wrong, it warps creatures into aberrations. But that’s not some fundamental place which lies behind all of reality, but a taint, a process, that is a part of reality. What you do matters, and the cthulhoids are, themselves, part of the same order of being as we are. A virus, perhaps, which attacks logic, or the response of reality to that virus.
My slaad and gibbering mouthers and flumphs are the allergic reaction to chaos, self-correcting bugs thrown off from the source code of reality in response to an incursion. They spontaneously generate in areas where astral taint touches the world (or its shadows). They mostly sort of “burn out”: while both creatures trash reality, they consume more than they convert, so they tend to starve themselves.
My beholders and aboleths and ‘flayers are all “spirits of taint”, creatures that are generated within an area of taint and escape from it. They’re metastatized, and stable. They wait, and plot.