Over at Wizards, Dungeon #188 has a new article by Robert J. Schwalb on one of my favorite abyssal residents, Juiblex the Faceless Lord (Yes, you will need a D&DI account to download and read it). This article has everything you need to battle Juiblex, his aspects and minions for 4th Edition D&D. Even if you don't play this edition there is plenty of lore to be mined here, including excerpts from the Demonomicon of Iggwilv:
"Undoubtedly, few denizens in the Abyss equal, in the horror they evoke or in the hideousness of their forms, the corrupt majesty of the Faceless Lord. In the gloom that enshrouds his realm, he holds court over the glistening masses, the twitching oozes he sires. Juiblex is deemed a lesser figure in abyssal hierarchy, judged Lord of Nothing by his peers. However, in my research, I have plumbed the foul mire to examine the full darkness that is this disturbing fiend. Juiblex is a power defined by incremental advances, a being whose wickedness flows slowly but inexorably toward an unspeakable end, which troubles even this writer."
In addition to monster information, Schwalb's article gives an overview of the history, terrain and denizens of the Slime Pits of Shedaklah, where Zuggtmoy the Queen of Fungi rules the surface while the Faceless Lord broods in the tunnels below. All in all this download is quite useful to anyone who makes heavy use of demon lords or planes, which are so heavily integral to the Greyhawk campaign.
Juiblex is also called He Who Slithers, the Unnameable One, Glistener, Indescribable Darkness, and Slick of the Foulest Humors. Speaking of humors, for those who don't have a subscription to D&D Insider, I wouldn't leave you all hanging with out some Juiblex love. Here is an old episode of the Graz'zt Show from 2009.
Update 4/8/2021: I had to remove a broken link to the #188 issue and article since 4E is no longer archived on Wizards.