Durin's Door

The moon now shone upon the grey face of the rock; but they could see nothing else for awhile.  Then slowly on the surface, where the wizard’s hands had passed, faint lines appeared, like slender veins of silver running in the stone.  At first they were no more than pale gossamer threads, so fine that they only twinkled fitfully where the Moon caught them, but steadily they grew broader and clearer, until their design could be guessed.”

Invisible letters and glyphs are a great fantasy trope and can make for really interesting and engaging props in your game.  In your world, the letters or symbols could be drawn in Mithril ink that is only visible by moonlight,  arcane magic that is triggered by word or touch, or simply disappearing ink that’s exposed by heat or chemical reaction.  It could be a map, a secret door, a spell, or coded instructions from the king.

Invisible ink markers seem to be hard to come by these days, but I was able to find a company online that sells them.  Unless you have a black light at home already, you’ll also need something like this mini black light keychain to reveal the ink.  Other than that, all you need is some paper (see tips below).  Draw up an invisible map or letter, hand it out to the players at the appropriate time, and watch them puzzle over this mysterious prop.  Don’t give them the light, of course, until they’ve determined how (in game) to make the letters appear.

Here are some tips:

• Use clean, bright white paper, for best results.  If you use colored paper the ink may not blend in completely.  Paper that comes with a pattern already printed on it (e.g. a scroll pattern) seem to work okay, but your mileage may vary.  It may not work well over printer ink (if you prink out your own scroll texture, for instance).
• The hidden letters are easier to see under blacklight when the room isn’t brightly lit.  A good excuse to turn down the room lights for atmosphere.
• Consider hiding the secret message on a page with other words or symbols drawn on it, like at the end of a handwritten letter, or on a map.  A handout with words or drawings on it is much less likely to arouse the suspicions of your players than one that appears to be completely blank (of course, if you want them to be suspicious, then go right ahead and do so).
• DMs that make their own 3D terrain can even use the markers on them to draw hidden doors, runes, or other magical features.  The ink dries clear on craft paint just as it does on paper.
• I wouldn’t recommend drawing on your vinyl battlemaps with an invisible ink pen as it’s unlikely to come off.  It should, however, work on Gaming Paper.