It seems I forgot to mention this campaign idea in my earlier post:
The basic idea here is to begin in Undermountain - probably using the 4E adventure Halls of Undermountain - and then expand into an adventure inspired by the excellent DDi article The Xanathar: Beholder Crime Lord of Waterdeep which conveniently provides several NPCs and brief lair descriptions that would be rather easy to convert into an outline for a campaign for the Heroic Tier.
That said, while the campaign could easily end once The Xanathar is defeated, my plan is to have the basic plot of the campaign involve stopping a slaving operation ostensibly run by The Xanathar... but the ultimate customers are the drow of House Xorlarrin who need slaves for sacrifices, especially spellcasters, to fuel the Demon Weave (cf Menzoberranzan: City of Intrigue). If the campaign continues after the beholder is defeated, I am hoping to create a high Heroic Tier/low Paragon Tier homage to D1 Descent into the Depths of the Earth, D2 Shrine of the Kuo-Toa and D3 Vault of the Drow that ends in Menzoberranzan rather than Erelhei-Cinlu.
However, as the wonderful cover art for the Scoundrels of Skullport expansion for the Lords of Waterdeep board game that I have appropriated for this campaign shows not only The Xanathar but Halaster and a mind flayer, I'm also going to have to include Halaster, or at least a clone, together with a mind flayer and its druuth (if not its whole sept) so that the picture makes sense in the context of the campaign.