One dungeon was full of undead, but the way I did it made it seem like it detoured into a horror campaign (without the danger) but seamlessly returned to traditional fantasy. It was an abandoned fort built into the mountainside that was held by human soldiers but goblins had besieged it and in the process inadvertently collapsed it. Knowing they were cut off from supplies the goblins left them to starve. The PCs pass through the ruins of the surrounding time, complete with destroyed homes and corpses bloodying a now stagnant fountain. The PCs scale the only remaining tower, pick the lock, and start to climb down the spiral staircase when it breaks and takes them down to the ground floor. They immediately notice that the entire fort is filled with smoke and the revolting smell of dead flesh. As the cross over scored planks, it breaks and they fall below into a pit of rotten meat, where it had been previously stored. After defeating the swarm of maggots and crawling out, they investigate the rest to discover that a) the noblemen had committed suicide early on and their possessions were guarded by an incorporeal undead, b) that a wounded and slowly dying man had written his last words in blood on the wall (lines from the Rime of the Ancient Mariner), c) the commoners had been sealed off in the fire and forced to survive on cannibalism, the "victor" of which had become a fiery ghoul, and d) the soldiers had died guarding their king and priest and remain vigilant even in undeath (this is where the treasure is). The PCs really enjoyed the different flavor, but were also happy to surface and forget it all.

Ones the PCs ran afoul with the law in a way I did not expect them too, so the next session was the half of them that got caught breaking out while the half that didn't broke in. It was a highly entertaining and actually fit really well with the story.