A long while back, I ran a suspenseful little dungeon as a substitute for one of the old Rod of Seven Parts segment quests. It consisted of a largely abandoned crypt wherein there was only one monster (wait for it), and one form of treasure (the third segment of the Rod of Seven parts).
A dungeon with one monster doesn't sound like anything special. Under most circumstances, one might expect the dungeon to be over with very quickly. However, this dungeon was designed in such a way that while they were navigating its difficult-to-transverse passages, which seemed to pay little attention to gravity, they were at various times attacked by tentacles extending from convenient holes set in the wall, confronted with horrifying yet sourceless visions, and otherwise harangued by the single lonely monster as it sat comfortably in the center of the dungeon. With the right monster and the right dungeon, this sort of setup is entirely possible (and I recommend it).
The pcs spent a good three hours picking their way through the dungeon before they finally came to a huge mandala-like rune that was etched into the stone ceiling with gold dust. At the center of the really cool-looking glyph (I drew it), inserted into the ceiling, was the segment of The Rod. After some Knowledge (Arcana) checks, it became abundantly obvious that The Rod was powering an exceptionally powerful containment glyph. When the PCs eventually screwed up their courage and pulled the glyph out, I dropped my 3.0 monstrous manual and revealed the first edition Deities & Demigods, and flashed them the beautiful illustration of Yogsothoth. I guess in subsequent printings the Cthulhu Mythos was removed for legal reasons. At the time of this session, Call of Cthulhu had not yet come out yet, so the PCs were absolutely horrified. The PCs eventually defeated the (heavily modified towards the weak end of the spectrum) creature once they had stopped screaming.
[I love conceptual dungeons, and I love The Big Reveal.]