I'm pretty sure someone had a setting document where they wrote up a world where all the bad things came true. I caan't find it though! Check out this thread and this thread instead.
My copy of "Spires of Xin-Shalast" has a section titled What if Karzoug Wins? on page 61. It details what happens if the Leng Device is shut down, and what happens if the device is operational. In the former case, Karzoug rallies a massive army of giants and pulls another, greater army from ancient Xin-Shalast. In the latter case there's . . . cataclysm on the order of Aroden's death and possibly even Earthfall, as Mhar is awoken and freed.