I was running a 5th edition campaign with 5 players who were about level 8, having progressed all the way from 1st level. The party had been asked to round up a series of magical artifacts to try and prevent the return of a terrible magical force. Fairly typical D&D fantasy hero stuff.
The party had recovered one artifact and was searching for the next one. They arrived at the entrance to the crypt and found a doorway cut into the side of a mountain. It was obviously evil, obviously magical, and likely dangerous. The Warlock sent his Familiar into the doorway which was obscured by some form of magical darkness. The instant the Familiar hit the darkness, it died. The party tried several different ways to dispel the darkness, all to no avail. Finally, they decided to step through the doorway, confident that I wouldn't simply kill them outright with a sphere of annihilation or something.
The party awoke to find themselves lying on a dirty floor inside a rough-hewn cave. A Cleric, who was an NPC that the party had rescued way back on their first adventure together, was there reviving them. He explained that he'd sensed something was wrong with them and so traveled for three days and nights, only to arrive to find them dead on the cave floor. Each had received a boon from a powerful mage on an earlier adventure, basically a one-time Death Ward, and that had enabled the Cleric to bring all of them back…sort of.
The Cleric explained that he was unable to restore them all to life and so had done the next best thing…he'd turned them all into Revenants. They were now trapped between life and death. Necrotic damage would heal them and Healing spells would hurt them. Their skin was a deathly gray pallor and their hearts did not beat in their chests. They would have to secure the artifact, a magical book of spells, in order for the Cleric to fully revive them.
What followed was a fairly typical dungeon-crawl with all the usual trimming like traps, Undead servants, a monster or two, and even a Mimic disguised as a treasure chest. The only difference was that they had to rely on the Cleric's Cantrips to heal them as none of them had any Necrotic spells. They had to be much more conservative with their HP and spells, rationing both carefully rather than leaping in with both feet. In the end, they faced off against the BBEG's lieutenant who was guarding the evil tome they were seeking. As soon as he was defeated, everything around them changed.
The entire dungeon, from the doorway to the throne room at the end, was an elaborate illusion. The darkness at the entrance inflicted a couple of points of damage, enough to kill the familiar and actually cause harm, just to be convincing. The Cleric was one of the evil lieutenant's minions, mimicking their NPC buddy. Even their death-like state was fake. None of them had even tried using normal healing magic on themselves, relying rather on the 'Cleric' to heal them with his Necromantic damage spells, which were actually Healing in disguise. If any of them had tried to test the illusion, they would likely have seen right through it.
It was the only time I was ever able to pull off so elaborate a scheme. The next several adventures started with one or more party members falling back on the old 'I disbelieve' trick to see if what they were facing was real. All of them are level 14 now and the game is on hiatus for various reasons. When we resume later this year, I may try to fool them again…just to see if their paranoia has died down.