Some West African kings were buried in a manmade hill. First the king and his treasures, along with some unlucky servants, were interred in a wooden dome. Then tons of earth was piled on the dome. Finally the new hill was covered with clay and fire-hardened. This was a difficult tomb to rob - although a narrow vertical chimney was left. (Why? For the soul to escape, as in Greek tombs?)
This chimney could make a good tomb/dungeon entrance. It's wide enough for one PC to squeeze down; then there's, say, a 30-foot drop to the floor.
What's to protect such a tomb against robbers? It's easy to climb down to the floor on a rope. Let's say that in the center of the tomb, right under the shaft, is a rune-covered seal; anyone standing on it is paralyzed and takes ongoing damage (no save!) until they're somehow moved onto a safe square.
I'd play it like this: PC 1 says "I climb down the rope." The DM doesn't say that PC 1 is paralyzed; he says, "OK, we'll get back to you" and asks the other PCs what they are doing. If they yell down into the tomb, they get no answer. Based on how long the PCs spend waiting and talking before they take action, the paralyzed PC might take one, two, or three rounds of ongoing damage. (A mean DM would secretly time the PCs' discussions and dithering and deal damage every 6 seconds of realtime, but try as I might, I just can't play D&D like a World-Class Jerkwad.)
Once a second character comes down the rope and sees a paralyzed PC on an obviously magical seal, they have the puzzle of how to move the PC without themselves touching the seal - and, ultimately, how to get into the tomb.
I like this trap because it requires a little creative thinking to circumvent, not just a few Thievery checks.
This trap shouldn't be too hard for a party of PCs to deal with, though it would be absolutely deadly to a lone adventurer. (Indeed, lying on the seal, under the feet of the paralyzed PC, are the bones of a few previous tomb robbers.)
I might actually have the PCs collect two halves of a macguffin in two such tombs, both armed with the same trap. The first time, the PCs would set off the trap and have the fun of panicking and trying to save their friend. The second time, the PCs would have the fun of figuring out how to get into the tomb without setting off the trap. (Can we drop a rock on the seal and step on the rock? Can we swing on a rope like a pendulum?) There's no need to enter a third tomb because by this time, all the creative thinking has been done.
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