Well, it may have been the first thing you thought of, but possibly Bastion of Broken Souls?
If you want them to have a huge dungeon that will gradually lure them in, and get progressively harder, and be so huge and labyrinthine that it will take them several sessions to clear it all (if it doesn't kill them), then I recommend Rappan Athus I, II, & III by Necromancer Games (note that those are all separate purchases). Make sure to stop by their website for the web enhancements to those adventures also!