I played D&D with my son, his two friends, and their father yesterday. We had a blast. And a total party kill.
It was one of those riddles that's tied to a trap. The riddle was an oldie:
You come upon two doors, guarded by two magical statues. One statue tells you, "You must choose one of these two doors. One leads to your safety, the other your death. You may ask a question about which door to choose, but beware: you may only ask one question and while one of statues always tells the truth, the other always lies."
Because you don't know which statue is the liar, the answer is to ask one of the statues which door the other statue would recommend. If you ask the truthful statue, he'll truthfully tell you which door the lying statue would recommend - but that door would be the one that leads to your doom. If, however, you ask the lying statue, he'll falsely indicate which door the truthful statue would indicate, so you again you'll be pointed to the death door. In both cases you take the opposite door to the one indicated.
But, the eight-year-old chose one of the statues at random and followed it's advice. The door opened, and I told them, "Something is clearly wrong as the door opens. Black tendrils shoot out from the door grabbing each of you. You are instantly pulled inside to your death.
The characters were standing on a causeway two stories above a ruined great hall swarming with Utah raptors ("long tails"). I could have given them a fighting chance and had the causeway collapse and let the survivors fight to the last. But I wanted to leave them (and it was dinner time) with the failure and a question of how they might re-think that one. We'll see what happens.