I once ran a dungeon for a long-time group of friends. Every room was comprised of a stone floor, walls that were painted to reflect some sort of terrain, some bookshelves full of scrolls and dusty books, and a number of NPCs that attacked on sight, whose expressions were described as "A mixture of hatred and burning jealousy."
Now, most of these NPCs were somewhere around level 1-5, and the PCs had absolutely no trouble dispatching them since the lowest-level character was level 10. But as the dungeon continued, the NPCs became stronger, and attacked more fiercely. And always with that same jealous, hateful glare.
Eventually, towards the end of the dungeon, it dawned on them: The NPCs weren't NPCs, they were the group's PCs from all the campaigns and sessions we started and dropped, or just plain never got around to doing. The walls were painted in the biome of the setting's starting location, and the books on the shelves (originally described as making absolutely no sense) were the notes from said campaigns.
Before the last NPC died, he spoke- The only one in the entire dungeon to do so.
"Do not celebrate, 'heroes'," It spat. "You will join us some day, in the planes of dust and darkness."
"You will join us, and we will be waiting."