MCDM’s Delian Tomb features a nice little procedural puzzle: deep in the tomb, you can open a secret door by answering the riddle, “If you are to keep this, you must first give it to me.” But the answer “your oath” isn’t what opens the door. Instead, it’s speaking the Delian oath itself, which the heroes would have encountered elsewhere earlier in the tomb. I suggest taking this a step or two further.

The heroes explore an old temple of the Lexinor Knights. Spirits of the knights of Lexinor bar the way to the inner sanctum, where treasures of the order still remain. To enter, one must speak the oath of Lexinor.

“I, [your name], swear to be without fear in the face of my enemies, defend the helpless, do no wrong, and learn the lore of Lexinor and keep it secret.”

Any character that utters that oath has actually taken it and receives a vision making it clear that they will suffer the wrath of Lexinor knights’ spirits if they fail it. You can give the heroes a warning about this in some form earlier. Perhaps an NPC says “Only a sworn knight of Lexinor may enter the knights’ most holy place.”

In truth, there might be some other way (fighting the spirits, for one), but one of the heroes might decide it would be pretty cool to become the only member of a lost order of knighthood (something that would presumably come with some small advantage). Of course, that would suggest that they should be reluctant to allow the order’s sacred objects to be melted down, but maybe there’s enough mundane coin to go around, and the character can keep the order’s sacred objects as his or her share. Such tricky choices demand role-playing and build character.