[Butz]

You could have like a cemetary, and pushing one of the gravestones reveals stairs down into a dungeon / crypt of sorts.
Since it's overrun with dark power, spells of restoration and healing, along with holy spells, are disallowed.
You could have like, normal monsters down there at first, but when you kill them, a zombified version of them rises up, or perhaps it only rises up if a certain monster is nearby, like a necromancer. After travelling through the crypt, you'll get to a large room, and a huge undead monster appears. It can summon zombies and skeletons to attack you. (Perhaps new types of skeletons too, with different weapons. Something ranged? )
Anyway, the monster would be the undead creature "Dave" from the bestiary. Kill Dave, and the exit appears, and you can leave. ^.^.
Maybe get a new weapon that looks like a zombie arm that you use to club people. *Damage +7, Attacktypes Poison, Ghosthit, Weight 5 Kg (It's only an arm after all. A decayed one at that.)

Well there's my thoughts...though I just want to see a monster named Dave. ^.^;