Check out the details here. It's all quite overwhelming, fortunately I stick to just table-top gaming. Here's some observations and speculations (possible spoilers below):
"Rage of Demons will transport characters to the deadly and insane underworld. Rumors of powerful demon lords such as Demogorgon, Orcus and Graz’zt terrorizing the denizens of the Underdark have begun to filter up to the cities of the Sword Coast. The already dangerous caverns below the surface are thrown into ultimate chaos, madness and discord. The renegade drow Drizzt Do’Urden is sent to investigate but it will be up to you to aid in his fight against the demons before he succumbs to his darker temptations."
First, there is already some predictable blow-back on online to Drizzt being the main driver in this story. I understand why he's involved from a marketing standpoint and if you're going to use the Underdark it would be glaring not to involve the #1 drow. So yeah, no big deal here. It's Forgotten Realms, it's what they do.
"Players of the tabletop roleplaying game can descend into the Underdark in Out of the Abyss, a new adventure which provides details on the demon lords rampaging through the Underdark. Partners such as WizKids, GaleForce 9 and Smiteworks will all support Rage of Demons with new products to help bring your tabletop game to life."
I'm all for good demon minis, can't wait. Now the rumors of multiple demon lords terrorizing the Underdark is indeed an over the top epic storyline. How over the top? For perspective, Greyhawk (as well as FR) has seen Graz'zt manipulating things before in addition to lesser demon lords like Zuggtmoy, and the prince of demons himself, Demogorgon once tried to screw up Oerth in Savage Tide, but even then the PCs had to take the fight to him in the Abyss. Taken literally this is a bigger threat than Elemental Evil and Tiamat combined. Now something or someone has to have brought the biggest, most notorious demon lords in all the Monster Manual to the Underdark (specifically Menzoberranzan). It's not like they get along after all. Here is my fearless guess (don't read on if you want to be surprised):
"To really get in the mind of Drizzt, fans will have to check out Archmage, the new novel by R.A. Salvatore, scheduled for release in early September."
A tie-in novel about Drizzt isn't surprising anymore, but the title is Archmage. This is key. It surely isn't about Elminster because that's Ed's character. Are there any notable archmagi in Faerun who deal with demons? There may be dozens, I have no idea, but Wizard's m.o. is to use existing high profile properties, and you know what that means. So my spoiler, educated guess is that the culprit behind this Rage of Demons storyline will be none other than Iggwilv!
Yes friends, Iggwilv is very Greyhawk but she is also a planar antagonist who isn't necessarily bound to one setting. She shows up in nearly everything Graz'zt gets involved in, she has the ability to summon demons like no one else in D&D history, plus, she is the author of the Demonomicon, another item that lends itself well to a story where multiple demon lords are being rehashed and researched. I do indeed hope this guess pans out. Motives aside, who wouldn't like to see the witch-queen Iggwilv destroy the Underdark and the Sword Coast? I may side with the bad guys on this one!
Update: Darn it I was already proved wrong when I found the blurb for Out of the Abyss:
"The Underdark is a subterranean wonderland, a vast and twisted labyrinth where fear reigns. It is the home of horrific monsters that have never seen the light of day. It is here that the dark elf Gromph Baenre, Archmage of Menzoberranzan, casts a foul spell meant to ignite a magical energy that suffuses the Underdark and tears open portals to the demonic Abyss."
Archmage Gromph?! Alrighty then. My idea is better, but that doesn't mean she won't make a cameo at some point...stay tuned this Fall folks.