I've been pondering using Illisk in my upcoming campaign. so while reading up on it from every source I can find (primary source being, of course, the write up in 'Edge of the Abyss'), i'm also pondering implications and theories. so lets start with the basics: Illisk is a dead world wracked by horrific hell storms able to tear unprotected humans to bits within a very short period of time. Scattered around the planet are megastructures that are, for all intents, hive cities built by an as yet unidentified xenos species. each city has racks upon racks of xenos corpses all of whom appear to be plugged into a possibly world spanning system of cogitators, which are still mostly operational. the cog boys are trying to figure it all out, and they've found all kinds of weirdness - everything from the corpse of some Xenos lord plugged into a cyber throne room to a buried vault deep within the planet that has 'constructs of metal given the form of the alien'.
ok, lets start with the obvious. I don't think Illisk is the homeworld of these aliens. the planet was stripped bare of any useable mineral resources. there don't appear to be any evidence that the planet was the place where these xenos were born and evolved. granted…I don't have any evidence to say illisk wasn't their planet of origin either but, nobody would gut their homeworld in such a coldly efficient manner or with such unity of purpose. it just doesn't fit, y'know? plus, the likelyhood that their home planet would have exactly the resources necessary to build these megastructures is pretty low. So I think illisk was chosen specifically by this race of xenos to build and power these stuctures.
the next most obvious question is : why build illisk in the first place? these cities were designed to plug damn near everyone in this race to a giant computer system. But why do that? were these xenos intended to be part of some computer? were they to be additional computational resources or is there some sort of xenos 'virtual reality' paradise running on those computers?
my completely random speculation is that Illisk was one of two things:
1. these xenos decided to build themselves a virtual paradise where everyone would have everything they ever wanted and could be an aspiring god. peace and unity achieved, wars ended, all desires fulfilled. then they ended up dying out while masturbating to endless porn simulations and xenos versions of Civlization V and Anno 2070.
2. illisk either was one of the original 'bio-forges' used by the Necrons to turn their populations into their eternal legions of murdering metal or it was build by a species that attempted to duplicate the bio-forging process but something went wrong. illisk could also have been a failed attempt by the Necrons to reverse engineer the process and they just left the place there and walked away in disgust at their failure.
that's basically what i've got right now for the planet. i'm leaning towards theory #2, but with the 'failed attempt at reverse engineering the process' option. that opens up some interesting possiblities that might be fun to play with in a campaign.