From the PDF introduction
A Web Enhancement for Fiendish Codex I: Hordes of the Abyss
Got demons? Well, you will if you pick up a copy of Fiendish Codex I: Hordes of the Abyss, which is packed full of information on demons and demon lords and how to use all this nastiness in your campaign. The chapter on demon lords mentions aspects of the demon lords as more common foes than the demon lords themselves, but few aspects are presented in the book since the designers put so many other useful things in that tome.
Well, that is where this little guide comes in. Over the next month or so, you’ll meet eleven aspects of demon lords, all suitable for mid-level campaigns. This first outing presents four: Juiblex and his rival Zuggtmoy, Kostchtchie the Lord of Wrath, and everyone’s favorite (or second favorite) Demogorgon. This version of Demogorgon’s aspect is different from that presented in the Miniatures Handbook, since it focuses on a different side of the Prince of Demons. Several of the demon lords could have aspects that are different than the ones you see here, because an aspect is one facet of a demon lord rather than a lesser version of the whole package. If you don’t like the aspect presented here for your demon lord of choice, choose a different "view" of the demon lord and make your own. For example, Juiblex has a few combat tricks, plus a few spell-like and ooze-related powers. If you want a more oozy aspect of Juiblex, take away the grappling and constriction powers and give it the create slime and summon ooze abilities. Additionally, each aspect has entries with suggestions on how to use the aspect in the EBERRON and FORGOTTEN REALMS settings.