Right now in my high level 16th level campaign, I've got essentially 3 groups working not necessarily together, but respecting each others goals, at least for now.
The Dimwalt Scramasax is a group of outcast necromancers lead by a lich. Or at least that's what the wizard who was overthrown by the "lich" thinks. Certainly the lich template was used, but then I added the 5 level Incantifier prestige class from the recent Dragon with the Lady of Pain on the cover.
Then we have the Cult of the Crippling Gaze. Or the Great Crippling Gaze. Otherwise known as Shax, from Armies of the Abyss. Basically started as a secret society for disaffected nobles, and was corrupted into a cult of psycho murderers. They are mortal followers of Shax the demon lord, who himself may or may not report to "the Old Ones".
The "Old Ones" originally were the Slarecians from Scarred Lands. That was a previous campaign and now the old ones are a combination of devil/demon/Cthulhuesqe type entities. If I had the Lords of Madness book I'd use it for the flavor and crunch, but I simply add a mind flayer here or there and the party gets the idea that the old ones are involved.
The Old Ones enforce a peace between the devils and demons, (there is no Blood War, except when demon tries to enter Hell or vice versa, then all bets are off). Yugoloths are mercenaries also used by the Old Ones.
Together these groups have taken over a celestial fortress that the players know nothing about, for the time being, but is essential for continued life, (it guards one of the life sustaining resources most beings require).