Here is another installment of exTRAPaganza, our Thursday over-the-top trap series.
A great treasure sits in the middle of an arcane circle. The circle appears to be a ward of some kind, but all tests will prove it harmless. If the treasure is moved, an extra-planar creature is summoned who will be quite annoyed. Also, the process of getting to the center of the circle will have disturbed it to the point that it can no longer contain the creature. If, somehow, they manage to get in w/o disturbing the circle, the circle will now not let out any of the inhabitants.