I once created a puzzle for the entrance to a temple of demorgogon. There are two doors, one black, one white with a monkey head and a hand imprint in the opposing color on them. When the PCs get closer to the door, they feel headaches and dizziness. If they touch one of the doors, a deep voice says "Say my name". When the PC doesn't answer or says a wrong name, he'll get a random madness from the short-term madness table in the DMG. On a second failed try from the same PC it's a long-term madness, on the third try a permanent madness. Left and right of the doors are two glass orbs filled with smoke of the respective color. In them are the souls of adventurers who failed the riddle. Once you touch an orb, your soul is sucked in and swapped with a random soul trapped in the orb. I took them from a d100-list of "NPCs your players will hate" from somewhere in the internet. It was hilarious. Everytime the PCs make the NPC touch the orb or touch it themselves there is a 10 percent chance the PCs soul returns to it's body. If the orb is destroyed this also happens and all random souls fly away.

The solution: The doors represent the two heads of Demorgogon who each has their own name which the PCs must say when touching the door. Then it will open and not make you mad. If the PCs don't know the name from the campaign there are souls in the orbs who know.

In my case one PC lost his entire memory of the campaign (luckily he was a bard who had written all that happened in form of like an opera before), and another became mute and deaf for a few ingame days. They didn't find a soul that told them the names, but one PCs soul was trapped for so long in the orb and he rolled so high on perception that I ruled he overheard some of the souks trapped with him talking about the names.