Some years ago, on another board and using another nickname, I posted the concept of a homebrew campaign setting, wherein the PC's world had fallen to the illithids, who had instigated wars between the humanoid races before crushing the thus weakened resistance and conquering the known continent (and presumably the whole planet as well). Some time later I came across the Unbroken Circle of Zerthimon, realizing that I, unwittingly, had copied the fall of People, and dropped my idea.
Now I'm toying with the idea, though, to work this concept out nevertheless, since I'm still intrigued with the idea of a world where humanity (and folks like dwarves, elves and halflings, who by their natures qualify as 'human', too for me) has been defeated and is forced to live in hiding and to fight for the very survival.
One thing, however, I was not sure about was, how the illithids would have managed their invasion. (Or, for that matter, how any interplanetary or interplanar power in D&D, like the githyankis and the harmonium manage the conquests of prime worlds).
My ideas for this invasion are:
- To build portals in secret locations of the world beforehand, through which the thrall forces are transported to the world from the centers of illithid power. Illithids seem to have enclaves in the subterran regions of most inhabited worlds, so this could be a plausible way to change a stereotypical medieval fantasy world into a mind flayer dominated nightmare.
-To gate the invading forces in. (Could prove heavily resource consuming, though).
- To Perform a vast landing operation from space. that sounds heavily science-fiction-like, but the illithids are apparently a major spelljamming power, so such a feat should theoretically be possible within the context of the inner logic of the D&D settings.
- Mix of all.
What are your thoughts? 
p.s.: I know this is not directly Planescape-related, but I figure that here are a lot of bloods around who have lots of information and good ideas about illithids, githyanki, spelljamming and the like.
)
Well in Dark Sun the Githyanki tried to invade the planet through a specially constructed 'nightmare gate' built under a mountain.I think the theory was to muster their strength under the mountain while drawing allies together and then sallying forth to conquer the prime nations. Whether or not this would work is anyones guess.
I imagine that the Illithids would be more subtle, gradually infiltrating cities and taking over the ruling classes, slowly bending the world's cultures to suit their needs. I could certainly see them stirring up dissent between two nations that they control, each demonising the other to the point where the people happily accept their Illithid sponsored 'saviours'. Keeping the wartime mentality going for a few generations could work well for feeding as well, every time someone goes missing just say that they defected to or were assassinated by the other side.
"We're making a better world. All of them, better worlds." - Anonomous Harmonium Officer