Let me start. First, I must say I love what you do, I've ran a few of the encounters you posted on Giant ITP (namely, the Forgemaster and the Goblin grenades), and they were pretty darn cool. Anyway, here's my request:
I have a 10th level party, although by then they'll probably be 11th level. The party consists of a Kothian (Dragonlance Minotaur) Barbarian/Fighter, an Elf Bard (going for Sublime Chord), a Human Paladin 2/Favored Soul 8 (going for Gray Guard) and a Dwarf Wizard 7/Fatespinner 3 (with, I must point out , an incredible 67HP). The party is rather low-op,
They have been chosen to put an end to an evergrowing evil cult. The cult is based around those green stone altars, which have two effects: (1) praying on them gives power to the cult's leader (and that's why they try to destroy them); and (2) being close to them for too long gives you mutations, some nice, some nasty. Naturally, the nature of the mutation affects your standing in the cult: if it's negative (lose an eye, blood won't clot, etc...), then it's seen as if the altar doesn't approve of you, but if your mutation is positive (get larger, get extra natural attacks), then you are seen positively. Furthermore, some receive really powerful powers: for example, one of the recurring villains is an ogre who received an extra pair of arms, wings and a breath weapon. These are, usually, made leaders rather fast.
The party is currently destroying the second to last altar, and the last one (the one I need a cool encounter for) is located in the Empire, the most powerful country in the world by then, and an expansionist, ridiculously religious country. Their main population is human, although they have a large amount of Kothian (Minotaurs from Dragonlance) given that they conquered the Kothian's lands.
I want that chapter of the cult to be set in the sewers of the capital, maybe somehow related to the Thieves' Guild... And, because they hit lv11, I want a Beholder somewhere in there, most probably at the head of either the cult or the Guild. While the cult has a lot of mutants (where I usually just take any monster and refluff it as a mutant), it also has many "regular" people without any idea of what the cult is doing.