The 5e playtest version of Isle of Dread encourages the DM to spindle, fold, and mutilate the original adventure. The module suggests (spoilers ahead) having NPCs who visit the jungle center of the island return as undead, possessed, or otherwise corrupted versions of their former selves.
I went with this idea: the natives of the friendly village warned that explorers often come back “wrong:” not undead, but with an imperfect memory of their former lives and with a cunning tendency to act normal until they found a way to kill as many of their fellow villagers as possible. Therefore, the villagers had instituted a password and security-question system for letting explorers back in the village.
When the PCs met some native villagers walking around in the jungle interior, they somehow forgot about these hints. They trustingly hired the natives as guides: furthermore, they insisted that the villagers keep watch at camp so that all the PCs could go to sleep at once (including the high elf, who insisted that “I don’t NEED to sleep, but I LIKE to sleep.”)
That night, the corrupted villagers tried to creep up to the sleeping PCs and slit their throats. Only a series of improbably high PC Listen checks prevented a sleeping TPK. The PCs killed the corrupted villagers and then finished their rest.
The next day, surveying the carnage, the PC druid had a horrifying thought. “What if the villagers were sneaking up to put mints on our pillow or something? What if we’re the ones that are “wrong”? They went through the checklist: do we leave piles of their bodies in their wake? check. Are we remorseless? Check: this was the second night in a row the PCs had gone blissfully to sleep among corpses of their own making. Do we have poor memories? Sure, they’d forgotten that natives in the jungle might be dangerous. Besides, PCs never remember any plot points from week to week.
For a while, the players seriously entertained the notion that I was acting as an unreliable DM narrator, and the players had “gone wrong” and were killing innocents. And I? I cursed myself for not thinking of it. If I’d planned it, and managed to pull it off, this could have been The Creepiest D&D Game Ever.
OK, I don’t 100% encourage you to try this trick in your own Isle of Dread run. It could go horribly wrong and really alienate all your players. But on the other hand, it could go horribly right. Either way, it’d be a memorable campaign, and I’d like to hear about it.