Several skeletons and humanoid remains hang from wrist shackles high up on the east and west walls of the first room. One skeleton dressed in badly tattered chain mail lies slumped beneath an empty pair of shackles. Suspended by a thick chain in the center of the room is a small cage, containing the remains of a small humanoid; perhaps a halfling or even a child. There is a door at the east end of the north wall. However, what immediately commands the eye is a massive sheet of slate that takes up most of the room’s northern wall; about 12 feet high and 25 feet wide. The slate is filled with a rough drawing of a pair of long, sinewy quadrupeds with talons and whip-like tails. The images appear to be sketched onto the wall in blue chalk.
The slate radiates magic, and close inspection reveals that it appears to have several enchantments on it of both illusory and conjuration nature. Attempting to erase the sketches results in failure; rubbing, washing, and even magical cleaning only succeed in slightly fading the images at best. Even if the images are removed through magical means, they reappear several minutes later.
The door opens to a 5 wide hallway that extends for a short distance before opening into another large room. The only other exit in this new room is a door in the opposite wall. However, two massive creatures, quite similar in appearance to the images depicted on the slate, inhabit this room. They block egress to the opposite exit and attack anyone who dares to enter.
More careful scrutiny would reveal that both creatures are conjurations of sorts. Neither will leave the room. If somehow forced out of the boundaries of the room, they will wink out to reappear in the center of the room a moment later. Both creatures are quite formidable; they should be quite difficult for the party to overcome in direct combat. Even if they are killed, the creatures will reappear in the center of the room several hours later, as good as new.
The key to safe passage through this latter room lies in the slate room; a physical manifestation of anything drawn on the slate will appear in the room containing the creatures. Although ink and paint will dribble off the slate, it can be drawn on with chalk. Anything drawn on the slate (other than the original creatures) will fade and disappear in eight hours, as will their manifestation in the adjoining room.
Any creature drawn on the slate will cooperate to attack anyone entering the second room. Hence, drawing creatures to fight the existing sketches will backfire. However, inanimate objects drawn on the slate will manifest as inanimate objects in the second room. Any item removed from the room instantly disintegrates into a white, powdery dust the moment it is carried or forced out one of the exits.
When the PCs attempt to draw on the slate, Id suggest presenting the players with an actual paper containing the image of these creatures (or whatever creature you care to substitute); have them draw on this. Ideally, you want to use an illustration that actually looks like a somewhat rough sketch rather than a well-detailed monster. I uploaded the image I used to http://www.strolen.com/art/displayimage.php?album=random&cat=1&pos=-89. You might want to have several copies of whatever sketch you decide to use.
Once enterprising PCs figure out the chalk, there are numerous ways they might neutralize the occupants of the room; far too many to list here. Some possibilities are that they might draw shackles on the creatures, draw large weights tying the creatures down, draw large boulders in the air above the creatures, or make some additions to the creatures; showing them disemboweled or with swords thrust through their necks.
You can play this as loosely as you want; if none of your players is much of an artist, you might want to allow lesser renditions of objects to work passably. Otherwise, you might want visual flaws in their artwork to appear in the actual manifestations as well. (The line you drew in the huge manacles turned out to be a large crack, allowing the creature to snap them easily.)
I doubt that any group worth their salt will have much trouble with this trick. However, in the event that you need to provide more clues, Id suggest the following. Careful scrutiny around the slate will reveal chalk dust on the floor beneath, and perhaps a couple smudges of chalk on the board itself. You might also describe the creatures themselves as having a chalky skin; when a PCs weapon strikes one of the creatures, a harmless cloud of blue dust rises from the monsters hide.
If you end up using this, I’d love to hear what your players come up with to get through this. PCs do the darndest things…